A Question of Notation: Revisiting Moonscript

Growing Up Nicely

Since the last time I reviewed Moonscript here it has matured nicely in ways that make it easier to use. Some nasty gotchas (like a[i-1] being misinterpreted) have gone away and there is better error reporting.

It has found its way into my toolbox for quick utilities and prototypes that I don't need to share with others. (That may change; my colleagues know Python, not Lua, and I suspect that they will find Moonscript easier to read.)

I hope to make the point that even people who use Lua and don't wish to bet on an 'experimental' language can benefit from learning a little Moonscript, since it makes an excellent notation for expressing Lua programs. In particular, its terse syntax is well suited to interactive exploration.

Get Interactive

Out of the box, there is no REPL, but writing a sufficiently-good one was not difficult. mooni was the result. It does not try to solve the tricky problem of when to start a block; you indicate this by ending a line with a backslash.

moon-article$ mooni
MoonScript version 0.2.3
Note: use backslash at line end to start a block
> ls = {10,20,30}
> ls
{10,20,30}
> m = one:1, two:2
> m
{one:1,two:2}
> for x in *ls \
>>  print x
>>
10
20
30

Moonscript works with tables in the same way as Lua, except that the more conventional colon is used for associative key-value pairs. You don't always have to use curly brackets for map-like tables (e.g. the assignment to m above). Since all statements in Moonscript can have a value, we don't need any special way to indicate that a expression is being evaluated. mooni also does pretty-printing of tables.

A language with no libraries is a no-starter, but a language which is essentially a new notation for an existing language starts off with a ecosystem. For instance, we have Penlight available.

> require 'pl'
true
> utils.split 'one two three'
{"one","two","three"}
> utils.printf "hello '%s'\n", 'world'
hello 'world'
> utils.import math
> cos(pi/8) + sin(pi/2)
1.9238795325113

Function calls don't need parentheses, except when you need to force a function call to bind with an argument - in that case, the opening paren must not be separated by space from the function. (A more formal way of stating Moonscript's semantics here is that the call operator has a much lower precedence)

This sensitivity to whitespace takes a little getting used to, since Lua has practically none, but the payoff is that most function calls require fewer keystrokes, which matters in interactive mode.

The first great thing about an interactive mode is that the beginner has a chance to try statements out one by one, and gets their rewards ('it works!') and punishments ('why did that not work?') in little incremental steps.

The second great thing comes from the fact that we are often beginners; testing out a new library, exploring an API, trying out one-liners before inserting them into thousand-line programs, etc. So even if you are a Lua programmer, using a Moonscript REPL will allow you to experiment with your Lua modules in a less tedious way.

The function notation is very compact, which makes a functional style more pleasant:

> -- 'return' is implicit here
> sqr = (x) -> x^2
> sqr 10
100
> -- functions of no arguments do not need argument lists
> f = -> 42
> f()
42
> add = (x,y) -> x + y
> -- partial application
> add1 = (x) -> add x, 1
> add1 10
11
> ls = List{'alpha','beta','gamma'}
> -- mapping a function over a Penlight list
> ls\map => @sub 1,1
{a,b,g}
> ls\filter => #@ > 4
{alpha,gamma}

The fat arrow is short for a function with an implicit self; @ is a shorthand for self.

Everything is a Value

The use of indentation to indicate blocks is now firmly associated with Python, so people with a Python background might feel superficially at home. But consider this:

> f = -> 10,20
> {f()}
{10,20}
> if 0 then 'ok'
"ok"
> if 1 > 2 then 'ok' else 'nope'
"nope"
> ls = for i = 1,3 do i
> ls
{1,2,3}
> [i for i = 1,3]
{1,2,3}

As in Lua, functions are first-class values, and they can return multiple values. (In Python there is the illusion of multiple return values, but really they are packed into a tuple and then unpacked in any assignment, which is a lot less efficient). If such a function is evaluated as the last value in a table constructor, all the values are captured. This is all standard Lua semantics. The if construct should come as a surprise to both Lua and Python users, since it returns a value. (The gotcha for a Python user is that 0 is not false; only false and nil evaluate as false in Lua)

for statements are expressions that collect their values into a table, only if they are in an assignment context. So they don't needlessly create tables! For this simple task, list comprehensions are better, but consider this example from the manual:

doubled_evens = for i=1,20
  if i % 2 == 0
    i * 2
  else
    i

(The general rule is that the block versions of if,for and while leave out the then or do keyword. There is no end keyword!)

The with statement works like the similar statement in VB or Pascal; a dot is needed to indicate the fields that will be set within the table. And it's no surprise that it returns that table as a value:

> with {} \
>>  .a = 1
>>  .b = 2
>>
{a:1,b:2}

This gives us a really cool way to write modules, because (again) the value of loading a file is the value of the last expression.

-- util.moon
with {}
    .greeting = (name) -> print "Hello ", .quote name

    .quote = (s) -> string.format('%q',s)
----------
> u = require 'util'
> u.greeting 'Dolly'
Hello     "Dolly"

(Note how we can use the dot for reading fields as well as writing them.)

Doing Programs Bit by Bit

require in Moonscript works just like in Lua, except it will load any *.moon files on the module path as well. But require is not so useful for incremental and interactive development, because it will only load once and cache the result. Which is why we will rather use dofile for this purpose - but the global dofile from Lua and only loads Lua scripts. It is easy to make a Moonscript-aware dofile using the built-in moonscript module.

> moon = require 'moonscript'
> dofile = (x) -> assert(moon.loadfile x)()
> u = dofile 'util.moon'
> u
{greeting:function: 0xfd9610,quote:function: 0xfbc140}

So now it's possible to reload a module and try out the changes. The finger-friendly syntax makes interactive use easier. If I had a module which controlled a robot, then Moonscript provides a nice command prompt:

> turn left
> speed 2
> obstacle -> speed -2

Now this isn't such an imaginary scenario. PbLua is a Lua port for the Lego Mindstorms NXT kit, and it has a Lua prompt. Getting Moonscript to work with pbLua does not require that the micro actually runs Moonscript! A custom mooni could translate everything into plain Lua and push that up, ditto for scripts.

This point needs emphasizing - moonc compiles Moonscript to Lua. The Lua environment that then runs that code could be stock Lua 5.1, 5.2, LuaJIT or whatever. The decision to use Lua as the intermediate language has given us a lot more flexibility in applications.

In mooni, if you want to see what Lua code was generated by the last executed statement, use this common expression of puzzlement:

> t = one:1, two:2
> ?que
t = {
  one = 1,
  two = 2
}

Making up New Constructs

For instance, it seems self-evident to most people that a modern language should have syntax for exception handling. Coating Lua's pcall in some convenient sugar is very straightforward:

try = (block) ->
    ok,err = pcall block
    if not ok then err\gsub '^[^:]+:%d+: ',''
    else nil, err

test = (foo) ->
  err,val = try ->
    if foo then return 2*foo
    print a.x
  if err
    print 'error!',err
  else
    val

print test nil
--> error!  attempt to index global 'a' (a nil value)
print test 2
--> 4

There is still an issue if the function did want to return nil, but I'll leave the resolution of this to any interested parties. (hint: use select)

This kind of thing has been possible in Lua for a long time now, but people get put off by the necessity for (function() ... end) here, and anywhere where we need a lazy way to get 'lazy evaluation'.

For instance, when working with many GUI toolkits it's useful to schedule an action to be run later on the main thread. This could be expressed as later 300,-> do_something(). GUI toolkits are all about firing events; for instance in AndroLua one can create a button and attach it to an action in two lines:

@button "Please Click Me!",->
    @toast "Thanks!"

The equivalent Java code is a lesson in how boilerplate obscures otherwise straightforward code, and explains why Java simply has to get lambdas to compete.

Moonscript's syntax can play nicely in the niche established by Ruby. For instance, this is a rakefile.

task :codeGen do
  # do the code generation
end

task :compile => :codeGen do
  #do the compilation
end

task :dataLoad => :codeGen do
  # load the test data
end

task :test => [:compile, :dataLoad] do
  # run the tests
end

And here is the equivalent lakefile for Lake

-- lakefile.moon
task = target

task.codeGen nil, ->
    print 'codeGen'

task.compile 'codeGen',->
    print 'compile'

task.dataLoad 'codeGen',->
    print 'dataLoad'

task.test 'compile dataLoad',->
    print 'test'

-- without any explicit targets, lake fires this ....
default 'test'

moonc lakefile.moon would creae lakefile.lua, which lake understands. If anything, the syntax is even cleaner - I've cheated slightly by passing dependencies to the targets as space-separated strings; they can also be written as tables like {'compile','dataLoad'} which is the internal representation anyway.

Imagine a hypothetical environmental monitoring system:

rule.too_hot -> temp > 37 and humid > 80
--- handle the rule asynchronously...
If.too_hot -> print 'get them out!'

Which suggests that if I were designing a DSL (Domain Scripting Language) for such a rule-based application then my users might find Moonscript easier than plain Lua. (Embedding Moonscript in an application is not difficult, since it's mostly Lua code with dependencies on LPeg and LuaFileSystem. The Windows binary has already compiled this all into a DLL.)

Tooling and Documentation

This is something that separates the goats from the sheep when evaluating a new language. Fortunately, leaf has provided editor support - the repackaged SciTE is a good choice for Windows users. You will probably have a better experience if you edit the configuration file `Options|Open Global Options' and put these lines at the end:

split.vertical=0
open.dialog.in.file.directory=1
statusbar.visible=1

Assuming that you do want other people to use your modules, it helps to have a documentation tool that understands the language. This simple List class has basic LDoc markup. Note the use of the @within tag to put the metamethods in their own section.

The output from ldoc -f markdown -S List.moon is here.

(This is all hot off the presses so you'll have to grab from the LDoc master. I'm considering whether metamethods should automatically be put into their own section by default.)

Differences and Distinctions

Moonscript is compiled to reasonably straightforward Lua, and its detailed semantics are a superset of Lua so it finds easily into the existing Lua ecosystem. The surface syntax is different, but comes from several design choices

• indentation is syntax - no more end
• : for key-value pairs in tables; purely map-like tables often don't need curly brackets
• line ends become significant, so commas are not needed in multiline tables
• function(self,y) becomes (self,y) -> or (y) => depending on taste
• function calls have very low precedence, hence less parens needed
• every statement can return a value; explicit return is usually not needed
• local-by-default means local is only needed to make scoping explicit
• there is sugar for list comprehensions, classes and extended assignment operators like += and *=. != is a synonym for ~=

In other words, it is a determined attempt to reduce the typing needed for common operations in Lua, at the cost of more rules. This makes it a good notation for interactive work, even if your work remains mostly in Lua.

Could a person learn Moonscript without knowing Lua? No reason why not, but it will require a good self-contained tutorial and there are more syntactical gotchas. It could make a good educational language, since there you do not necessarily want a language that some of the class already know; familiarity breeds conplacency, if not actual brain damage (as Dijkstra asserted about Basic.)

Moonscript is available using LuaRocks or Windows binary - On Unix, sudo luarocks install mooni will bring in Moonscript as well, since it's a dependency. mooni itself is a single Moonscript file and can be found here.

Building Programs with Lake

Carpenters Bitching About Tools

One of the irritating things about programming is the set of available tools for building software. A lot of the appeal of dynamic languages comes from the simple fact that you don't build, you just run. A great deal of the fuss is accidental complexity, which is Fred Brooks' term for the gap between the complexity of the task and the actual complexity of the solution.

In the begining, there was Stu Feldman's make. People have subsequently wondered what deep reason there was behind the need for tabs, but the referenced quote from The Art Of Unix Programming shows us that it was an unhappy accident:

Why the tab in column 1? Yacc was new, Lex was brand new. I hadn't tried either, so I figured this would be a good excuse to learn. After getting myself snarled up with my first stab at Lex, I just did something simple with the pattern newline-tab. It worked, it stayed. And then a few weeks later I had a user population of about a dozen, most of them friends, and I didn't want to screw up my embedded base. The rest, sadly, is history.

make is a fantastic tool, firmly in the Unix tradition of doing one thing well. It runs commands when any of their inputs are more recent than their output. Its power comes from all the other powerful little tools that come with a traditional Unix environment. (This naturally becomes an issue on Windows, where you basically have to mimic that environment closely enough, for instance MSYS.) Its pain comes from having to cope with all the weirdness of different systems, even within the POSIX world, with tools of inadequate expressiveness and power. In other words, it's a bad programming language.

There are many alternative build systems which came out of this frustration. And an entertainingly profane plea on Reddit to simply stop building new ones.

Human nature being what it is, it did not stop me. I have no particular great hopes of it gaining any fans, but Lake does provide a working example of how Lua is suited for embedded Domain Specific Languages (DSLs).

Using Lake to cope with the C

Say we have the canonical first program, hello.c. Running the lake command works as expected; there must be a lakefile:

 c.program 'hello'

and then

 $> lake
 gcc -c -O1 -Wall -MMD  hello.c
 gcc hello.o  -o hello.exe

Running lake again will give the message 'lake: up to date'. If hello.c changes (or we deleted hello.o) then things will rebuild.

This seems fairly underwhelming at first, but then we knew this was a trivial program to build in the first place. Now, if Lake finds the Microsoft command-line compiler cl.exe on the path, then it changes its tune:

 $> lake
 cl /nologo -c /O1 /WX /showIncludes  hello.c
 link /nologo hello.obj  /OUT:hello.exe

(This is what will happen on Windows if you execute Lake inside a Visual Studio command Prompt)

The whole idea about lakefiles is that they express the build on a higher level, and let the tool decide on the incantation. This is particularly useful if you aren't familiar with cl.exe, for instance.

But there is more. This simple lakefile provides:

• cross-platform, compiler-agnostic builds (On Unix it knows to drop off the '.exe')
• an automatically generated 'clean' target, so lake clean will do its job
• a debug build by saying lake -g or lake DEBUG=1

I do work on embedded Linux sometimes. If I wanted my hello to work on a Gumstix then the incantation would simply be lake PREFIX=arm-linux and the correct compiler and linker would be invoked.

OK, let's get more fancy. The hello program has a second file utils.c and a shared header common.h. The lakefile now looks like this:

 c.program{'hello', src = 'hello utils'}

Please note the curly braces: program is a function of one argument, which is here a Lua table constructor. You can put parentheses around the table, but it isn't required. The sources are provided as a simple space-separated list of names; Lake already knows that the extension must be .c.

This build works as desired; if any of the two C files change then they will be recompiled, and the program linked. Lake knows that hello.exe depends on hello.o and utils.o, and it knows that these in turn depend on the corresponding source files. But it even knows that compiling the source files depends on the shared header - so that editing common.h (or just updating its timestamp with touch) will cause both of them to be rebuilt. Both of these compilers can be told to show what include files they depend on, using the -MMD and /showIncludes flags respectively. Lake uses this output to add extra dependencies to the compile rule for the files. Managing the dependencies manually is irritating, and easy to get wrong.

Say utils.c had a reference to sqrt. The lakefile should now be:

 c.program{'hello', src = 'hello utils', needs = 'math'}

Now for Unix builds, the math library will be linked in with -lm; on Windows, this is unncessary since the runtime already includes the math library.

Everyone has Needs

Lake uses this idea of needs for builds to specify their requirements on a higher level.

pkg-config is a marvelous utilty that provides exactly what is required here. Unfortunately, it is not used widely (or consistently) enough to be a one-stop shop for providing the gory details about every library. But Lake will try to use pkg-config if it is available to match needs. So a simple GTK+ C program can be built like so:

 c.program{'button',needs = 'gtk+-2.0'}

Lake resolves needs like this: first, whether it is built-in (like 'socket' or 'lua'), second, whether there are pkg-config aliases like 'gtk' available, and third, whether suitable global variables have been defined. So in resolving the unknown need 'foo' Lake will see if the globals FOO_INCLUDE_DIR, FOO_LIB_DIR and FOO_LIBS have been defined and point to existing directories. (Thereafter, it will try pkg-config.)

A lakefile is ultimately just a Lua script, and can have code that sets these variables explicitly.

 FOO_LIBS = 'foo3'
 if WINDOWS then
     FOO_DIR = 'c:\\foolib'
 else
     FOO_INCLUDE_DIR = '/usr/include/foo3'
 end

Explicit Rules: running Tests

Say I have a little Lua C extension. That's straightforward because Lake knows about Lua:

 mylib = c.shared{'mylib',needs = 'lua'}

Now I wish to run some Lua test files against the generated mylib.so or mylib.dll. For this, we make an explicit rule that connects Lua source files with an output file like so:

 lt = rule('.lua','.output','lua $(INPUT) > $(TARGET)')

 // populate the rule with targets; it depends on mylib
 lt ('test/*',mylib)

 // the default target depends on both the library and the test targets
 default{mylib,lt}

One important take-home here is that Lake works with targets in a very similar way to Make; the first target defined in a lakefile becomes the default, but if there are multiple targets then we have to define a dummy target that depends on these targets.

Now, maybe there is also a requirement that tests can always be run directly using lake tests. So we have to create a target dependent on the test targets, which first resets the tests by deleting the fake targets:

 target.tests {
   action(utils.remove, '*.output'),
   lt
 }

Depending on an unconditional action does the job. (However, this is not entirely satisfactory, since in an ideal world the order of dependencies being resolved should not matter, but this will do for now.)

Making the World a Better Place, one Semicolon at a Time

I remember an entertaining article by the famous Verity Stob on what to do when encountering C++ errors. One of the options was to write a Perl script to unmangle the errors so that they could be read by humans. She was writing satire, but like most good humour it was more than just a joke.

For instance, here is a wrong C++ program. Not terribly wrong, in fact almost competent:

 // errors.cpp
 #include <iostream>
 #include <string>
 #include <list>
 using namespace std;

 int main()
 {
   list<string> ls;
   ls.append("hello");
   cout << "that's all!" << endl;
   return 0;
 }

The response is pretty scary:

 errors.cpp:9: error: 'class std::list<std::basic_string<char,
 std::char_traits<char>, std::allocator<char> >,
 std::allocator<std::basic_string<char, std::char_traits<char>,
 std::allocator<char> > > >' has no member named
 'append'

Seasoned C++ programmers learn to filter their error messages mentally, but this is the kind of initial experience that drives kids to sniffing glue.

lake provides the ability to filter the output of a compiler, and reduce irrelevant noise. Here is the lakefile:

 if CC ~= 'g++' then quit 'this filter is g++ specific' end

 lake.output_filter(cpp,function(line)
   return line:gsub('std::',''):
     gsub('basic_string%b<>','string'):
     gsub(',%s+allocator%b<>',''):
     gsub('class ',''):gsub('struct ','')
 end)

 cpp.program {'errors'}

And now the error is reduced to:

 errors.cpp:9: error: 'list<string >' has no member named 'append'

And another case of rampant template trickery gone bad has been tamed, and our hypothetical beginner gets to Nirvana quicker.

This was, incidently, an accidental feature. I needed to parse the output of cl.exe to get the header dependency information (it is not written to a .d file like with gcc) so a postprocessing hook was needed.

What Next?

Naturally, this is not a new idea in the Lua universe. PrimeMover is similar in concept, and also has a bootstrap stage to construct a completely self-contained interpreter, which is definitely a strategy worth emulating.

I haven't dealt with topics like dependency-based programming because this is not intended as a manual (which is to be found here) This article is more about showing the advantages of a higher-level, needs-based build system based on a real programming language, which is compact enough that a fully self-contained Lake executable would be less than 300K.

A number of kind people have pointed out that 2,500 lines of code is a bit much for a single script, which is true, and of course I know better. Unfortunately I have too many projects and they keep me awake at night, demanding to be fed; the next evolution of Lake will have to take its turn.

A single file does make installing Lake easier; it just needs Lua and LuaFileSystem (known as the lua5.1 and liblua5.1-filesystem0 packages in the Debian/Ubuntu world) and for the script to be made executable and put on the path. If you have installed LuaRocks (also available on Debian/Ubuntu) then installing Lake is as simple as sudo luarocks install lake.

The priority is a sound system that is flexible enough to meet working programmer's needs, to get the right balance between declarative/dependency-driven and imperative. It is already possible to provide new needs for Lake by defining Lua modules that look like 'lake.needs.NAME', which can then be easily installed by LuaRocks or some more ad-hoc delivery system.

Using all the processing cores that developers have available is also a priority, which requires some interesting work in a language that does not do the necessary concurrency out of the box. The best cross-platform candidate would be Lua Lanes which provides a non-shared concurrency model with explicit data messaging using 'Lindas'.

Documenting Lua

Why is Documentation so Lacking?

As Barbie the Software Engineer might say: Documentation is Hard. It's not intrinsically fun like coding, and often gets written badly as an afterthought. In open source projects, where fun is so often the guiding principle, it can get badly neglected.

There are basically two kinds of comment; one that is aimed at other developers of the code and the other which is aimed at the users of a library. The latter is particularly hard and important, since we are building abstractions for other people to use, and if they have to actually read the code to work out how to use the library, that abstraction has broken. This is a common problem with language bindings to existing libraries; they will be announced as a 'thin wrapper around libfoo' and it's assumed that you can work out how to use the bindings from the libfoo documentation or from libfoo examples in C. Well, not everyone can (or needs) to read C well enough to make that practical. We know the classic rebuke, "RTFM!" but that assumes that there is a manual - it's often "RTFS!" these days.

Testing has become a minor religion in the last ten years, and mostly it's a good thing, especially when using dynamic languages. Tests are supposed to be the best kind of documentation, because they can automatically be shown to be correct. But if the scope of the tests is too narrow, then they can be harder to read than the source. That is, also write examples that can serve as tests.

We cannot ascribe poor documentation to laziness, since these projects are otherwise well-constructed and may have exhaustive tests. The developers clearly don't think documentation correlates with wide use by the community; perhaps communication is not a priority. After all, research mathematicians write papers and do not immediately think how to explain it to undergraduates; they are writing for peers.

This might seem an indirect criticism, but it isn't intended as such. (I resisted the urge to point to some representative Github projects for this reason.) The Open Source universe needs all kinds, from researchers to popularizers. In the 'social coding' universe, fork has become a verb of respect, so documenting an existing project and doing a pull request is a polite way to contribute to the usefulness of a project.

You might think that elaborate conventions and markup could get in the way of documentation. The Go standards simply require you to preface the package and any exported items with a plain comment; just about the only convention is that indented lines are shown preformated. And still those plain comments are often not written. Part of the problem is a variant on the "increment i by one" kind of comment, which is just an awkward paraphrase of the code. So a Stack.Push method might end up with the blindingly obvious "Push an element onto the stack". Anybody with any self-insight is going to find that silly, and so they don't do it. I have noticed that coding libraries and documenting their interfaces are modal activities; it's useful to approach documentation with a fresh head (much as how writing and proof-reading are separate but essential steps in preparing an article.)

The key thing is to care enough to return to the code with "beginner's mind", and finish the job.

The xDoc Tradition of API Documentation

APIs are rarely as self-documenting as their creators might think. Javadoc is probably the grand-daddy of a per-function style of API documentation, and it has inspired numerous xDoc tools for other languages. Here is an example of LuaDoc markup:

 --- foostr stringifies a list of Foo objects.
 -- @param foolist a list of <i>Foo</i> objects
 -- @param delimiter string, as in table.concat
 -- @return a string
 -- @see Foo, table.concat
 function foostr(foolist,delimiter)

Limited code inference occurs, so that we do not have to specify the function name, but otherwise everything is explicit. Dynamic languages pose a particular challenge here because we're forced to specify the parameter types in an informal way. Projects may define their own conventions, but there's no standard way to write 'list of Foo'. You may use see-references to other documented entities but they have to be in a @see section.

People have recognized for a while the limitations of this way of documenting APIs, which focuses on the individual pieces and can be silent on how to put them together, but it's much better than nothing. So the debate is often about making these comments easier to write. For instance, in a comparison between Doxygen and JavaDoc, Gushie says:

The most important problem with the Javadoc comment in the comparison is how much I need to concentrate on formatting issues while writing it. When writing Javadoc I am constantly thinking about what should and shouldn't be linked, whether the list will look right, etc. This is frustrating because, while I do want to document my code well I also want to focus on coding.

And that's it: a tool that forces you out of code flow is not going to make you happy about documentation.

One of the problems I have with LuaDoc is that HTML is awkward to type and ugly to look at, especially when you want to provide something as simple as a list. So one of the main requirements was that Markdown could be used instead. I try to be a good boy and not reinvent too many wheels, but LuaDoc proved fairly resistant to this change, since it helpfully stripped out the blank lines and line feeds from the comments at an early stage. But the tipping point was when I moved the Penlight codebase away from using the module function; LuaDoc loves the module function and it's often necessary to put it in a comment so that it will recognize something as a module. So ultimately the issue was that LuaDoc wanted code to be written in a particular way; futhermore, we all know that 'Lua does not have classes' but it's straightforward to work in an OOP style; again, there was no provision for such an choice. The guiding principle should be: a documentation tool should not limit a developer's choices.

So, wheel-reinvention happened, leading to LDoc. In a few days, I had something that did the job equally well, for myself. Seasoned campaigners will know of course that this is the easy bit; the hard bit is getting a tool that will work on other people's codebases. Thanks to some painstaking testing by Lorenzo Donatti and others, LDoc is now at the point where it's ready for prime time testing.

The lack of inline entity references in LuaDoc means that your references have to be footnotes to your comments. This style felt better:

 --- foostr stringifies a list of Foo objects.
 -- @param foolist a list of @{Foo} objects
 -- @param delimiter string, as in @{table.concat}
 -- @return a string
 function foostr(foolist,delimiter)

Using Markdown helps formatting lists:

 --------------
 -- creates a new @{Boo} object.
 -- @param spec a table of:
 --
 --  - `age` initial age of object (optional, defaults to 0)
 --  - `name` descriptive name used in caption
 --  - `foolist` list of @{Foo}
 --
 -- @return @{Boo}
 function Boo:create(spec)

(The blank lines are somewhat irritating, but I chose eventually to work with standard Markdown here; it is certainly less awful than the equivalent HTML.)

This kind of 'ad-hoc' table structure is a common pattern in Lua, and I've argued before that it's a good idea to have named types so that documentation can refer to them. An in-between option is to create a dummy type which just exists for documentation purposes.

Plain and Simple

Not everyone likes this 'tag soup' style of documention. For instance, this is the style followed in the Lua manual:

 ---
 -- Receives zero or more integers. Returns a string with length equal to
 -- the number of arguments, in which each character has the internal numerical
 -- code equal to its corresponding argument.
 -- Note that numerical codes are not necessarily portable across platforms.
 function string.char(...) end

This can be very effective with documenters who have a good clear prose style.

LDoc can support this by making the usual 'Summary' and 'Parameters/Returns' sections in the HTML template optional.

It was always awkward to configure LuaDoc, so I felt that LDoc needed a configuration file. As is often the case, Lua itself is an excellent format for such files.

 -- config.ld for Lua libraries
 project = 'Lua'
 description = 'Lua Standard Libraries'
 full_description = [[
 These are the built-in libraries of Lua 5.1

 Plus documentation for lpeg and luafilesystem.
 ]]
 file = {'builtin',exclude = {'builtin/globals.lua'}}
 format = 'discount' -- use lua-discount for formatting
 -- switch off the top summary and 'Parameters/Returns' in the template
 no_summary = true
 no_return_or_parms = true

(You can in fact even define your own template and/or stylesheet and reference them in the config.ld file.)

And here is the result, thanks to the API files from mitchell's TextAdept editor project.

The principle followed here is: allow the documenter as much control as possible, but make it straightforward for end-users to build the documentation. So a simple invocation of 'ldoc .' in the right directory will find the configuration file and do the rest.

Type Annotations

On the other end of the scale, it's useful to have type annotations for Lua functions. This is a requirement for full IDE support in the Eclipse Koneki project. Fabien Fleutot provided support for tag modifiers, so that one could say things like @param[type=string] name. I have provided convenient alias tags so one can say:

 --------------
 -- list available servers providing a service
 -- @tparam string service name
 -- @treturn {Server,...} list of servers
 function list_available(service)
 ...
 end

Beyond the built-in Lua types ('string','number',etc) there is no agreed-upon way to specify types, so some invention is necessary. The convention is that {T,...} is a list-like table containing T objects; (see the documentation for further discussion.) LDoc will attempt to do lookup on all identifiers within such a type specification, so that Server becomes a link to that type's definition.

Examples, Readmes and Command-Line Help

Per-function API documentation is often not enough. In particular, good examples can clarify the intended use, and narrative documentation can introduce the concepts and fill in the background. Complementary 'modalities' of explanation and extra redundancy allow end-users of different backgrounds and thinking styles to have various ways of understanding the topic.

So LDoc allows integrating the API documentation with examples and Markdown text as linked documentation. The best example I currently have is the winapi documentation. This is actually a C Lua extension, which is another LDoc feature. Here is the config.ld:

 file = "winapi.l.c"
 output = "api"
 title = "Winapi documentation"
 project = "winapi"
 readme = "readme.md"
 examples = {'examples', exclude = {'examples/slow.lua'}}
 description = [[
 A minimal but useful binding to the Windows API.
 ]]
 format = 'markdown'

Those of us who live in the command-line like to look up documentation quickly without messing around with a browser. Generating man-pages is an interesting and very do-able goal for LDoc, but man is showing its age and is pretty Unix-centric. Some great discussions with Norman Clarke inspired an LDoc feature where it looks up libraries on the module path and parses doc information.

So if you have an installed library with documentation, then it's easy to look up a particular function:

 $ ldoc -m pl.pretty.dump

 function        dump(t, ...)
 Dump a Lua table out to a file or stdout.

 t        {table} The table to write to a file or stdout.
 ...      {string} (optional) File name to write to. Defaults to writing
                 to stdout.

Thanks to mitchell's work at luadoc-ifying the Lua manual, this works for the standard Lua libraries (plus lfs and lpeg) as well:

 $ ldoc -m lfs.currentdir

 function        currentdir()
 Returns a string with the current working directory or nil plus an error
  string.

(ldoc -m lfs would list all the available functions.)

Further Work

It's possible to process the inner representation generated by LDoc using a custom Lua module, allowing multi-purpose pipelines. We're still working on use cases for this, but the basic machinery is there.

Other output formats could be supported; the program is meant to be extendable and I think generating LaTeX could be an excellent way of getting good-quality output.

Lua is surprisingly flexible at expressing concepts like modules and classes, so simple lexical code inference is probably not going to scale. Eventually we will have to use full static code analysis like David Manura's LuaInspect.

MoonScript: A Lua-Based Language

Let a Thousand Flowers Bloom

There are literally hundreds of programming languages, most of which are historical curiosities, research tools or personal projects. You would think that most of the important innovations have happened, and existing mature tools available for all tastes. That is true, to a point: but we are still exploring the space of expressive notation for solving our programming problems. Here the practical programmer and the academic researcher diverge, of course; the latter sees most new languages as rehashes of things first done in the golden age of PL development.

The last few years has been a surprisingly fertile time for new programming languages. They range from Go (native code + new runtime), Scala (bytecode + JVM) to languages like CoffeeScript or Dart which translate to JavaScript. The trend is mostly towards static typing but with type inference helping to keep the syntax clear and uncluttered, with functional influences. There is some TIOBE and anecdotal evidence that the dynamic language usage has peaked, possibly because of the maintenance difficulties of large codebases (an explicit motivation for Dart's development, which has optional typing.)

MoonScript is inspired by CoffeeScript, but generates Lua code instead. JavaScript and Lua share many similarities like first-class functions, the m["name"] == m.name equivalence, and no conventional OOP constructs. People like the convenience and familiarity of class, however, so CoffeeScript, Dart and MoonScript provide explicit support.

Trying it Out

When reviewing a new language, it's useful to remember that programming languages take time and exposure to become mature. MoonScript is now on its second release, three months after the first, and getting out of its proof-of-concept phase. 'Leaf' Cocoran is doing all the right things; he has provided syntax files for some common editors, a binary for the notoriously compiler-shy Windows community, and has a good looking website with comprehensive documentation. And he is building on a very solid base, the battle-tested Lua 5.1 release.

Also, reviews are not tutorials; I won't try to cover all the syntax, but will try to give the flavour of the language.

MoonScript could generate Lua bytecode directly, but would hit the basic issue that the "Lua VM" does not have standardized bytecode. In particular, this would exclude MoonScript from using the excellent LuaJIT implementation, which is a known problem with the otherwise powerful Metalua dialect. And having a readable high-level language as the compilation output makes it easier to develop and trouble-shoot.

Although not compiler-shy, I am lazy and just downloaded the Windows binary (it's hard to resist a language delivered as a 212KB zip file. Besides, the 'non-POSIX' world doesn't get enough attention these days.) If you want to avoid the 'moon script.moon' incantation, then it's straightforward to tell Windows how to handle MoonScripts:

 ftype moonscript=d:\utils\bin\moon.exe %1 %*
 assoc .moon=moonscript

(These make global permanent changes, so you need to run as Administrator). Then add .moon to the system PATHEXT variable. The incantation is now just 'script'.

 -- script.moon
 print "Hi Ma", "No Parentheses!"

Wish-list Driven Design

In many ways, MoonScript is like a collection of features from a Lua user's wishlist; list comprehensions, shorthand for lambdas, switch statements, explicit classes, statements-as-expressions, switch statement, local-as-default. But Lua is the C of the dynamic languages; it has a small, very powerful core and 'conventional' syntax which has made it popular as an extension language. It moves slowly and carefully, and does not pander to 'Python envy'. MoonScript, on the other hand, will appeal to people with Python exposure, since indentation is used in the same way to delimit blocks.

I would not recommend MoonScript to a beginner in the same way I could for Lua itself, since it has a very concise syntax that appeals more to seasoned professionals. There is a lot more to remember, and always a number of ways to do similar things. This example shows the conciseness of expression:

 sqr = (x) -> x*x

 print sqr 4, sqr 5

The last statement in a function becomes its value; there's still return available but usually it's only needed explicitly when leaving a function before its end. print is still the same function it is in Lua, but parentheses are usually optional.

Well, cool: we've saved a lot of typing. That translates to something like this:

 local sqr
 sqr = function(x) return x*x end

 print (sqr(4),sqr(5))

Now saving on typing is an ambiguous virtue; if we wanted the ultimate compression, we would all be using APL and playing Code Golf. Classic Lua programmers prefer keywords to sigils, and fear that their language would end up like Perl. They say that reading code is a much more common operation than writing it. And that is fine and dandy (I mostly agree), which is why we need projects like MoonScript and Metalua to get a feeling for new notation.

Personally I like how MoonScript makes the functional aspects of Lua stand out more clearly. Higher-order functions are less cluttered with keywords:

 compose = (f,g) ->
   (...) -> f g ...

 printf = compose print, string.format

 printf "the answer is %d, '%s'", 42, "Everything"

List comprehensions are another favourite functional idiom:

 range = [i for i = 1,10,0.1]
 squares = [sqr(x) for x in *range]

Note the * syntax, which satisfies the common need to iterate directly over the elements of an array. (You can still say for _,x in ipairs range) These constructs result in inlined Lua code, which is a good decision given the very fast translation times for Lua and its modest resource needs.

Multiple for clauses are allowed:

 points = [{x,y} for x in *x_coords for y in *y_coords]

That generates about 17 lines of Lua output; hand-written would be about 10, which would still slow readers down and make them have to guess the programmer's intention.

Statements can be used as expressions, which is useful in functions:

 choose = (c,a,b) -> if c then a else b

In fact, there's a clever feature that for statements return tables:

 show = (t) ->
     for k,v in pairs t
         print k, v

 res = for i = 1,10 do 2*i

 show res

This is sufficiently clever to cause unintended consequences, so the rule is that this does not happen in function expressions and cause needless table generation:

 loop = -> for i = 1,10 do i

 print loop()
 -->
 (nada)

It's probably too clever to be truly wise, given that list comprehensions do the job more explicitly. However, it can be used to go beyond the list comprehension syntax:

 values = {1,4,3,5,2}
 v = for v in *values do switch v
     when 1 then 'bad'
     when 4 then 'cool'
     when 5 then 'excellent'
     else "uh-huh"
 -- v is now {'bad','cool','uh-huh','excellent','uh-huh'}
 --

New with this release are table comprehensions, which are a generalization to map-like tables. These use curly braces to distinguish themselves, and the expression must be double-valued, with the first value becoming the new key and the second the associated value. So this expression turns an array into a set, that is t["hello"] is true if ls contains the value "hello".

 t = {k,true for k in *ls}

At this point, it's good to remember that we are still in Lua land, and there is no automatic pretty-printing of tables - which is wise because tables are the ultimate flexible data structure. (You can choose the string representation of your own types easily enough.) There is a small library included which provides a generic table dumper:

 require 'moon'

 moon.p {
     one: 'hello',
     10,
     20
 }
 -->
 {
     [1] = 10
     [2] = 20
     [one] = "hello"
 }

(For those used to Lua, note the use of : for key-value assignments in table constructors; the key may also be a keyword. For those who are not, note that tables can both be 'arrays' and 'hashes' - they are general associative arrays which are particularly efficient at regular array indexing. Like electrons, they are both waves and particles.)

However, despite the perception, there are batteries for Lua and MoonScript can consume them happily.

Using Existing Lua Libraries

MoonScript already includes LuaFileSystem and LPeg as standard libraries.

 total = 0
 for f in lfs.dir "." if string.match f "%.lua$"
   size = lfs.attributes(f).size
   print f, size
   total += size

 print 'total',total

(It also comes with lua-getopt for argument parsing, but that's an awkward, no-documentation kind of library.)

My favourite command-line parser is Lapp, which is part of the Penlight suite of libraries, since it parses the actual flag names and types from the usage string:

 -- mhead.moon
 require 'pl'

 args = lapp [[
     A simple head utility
     -n,--lines (default 10) number of lines to print
     <file> (default stdin) file to print; can be stdin
 ]]

 for i = 1,args.n
     line = args.file\read!
     if not line then break
     print line

On Windows, the directory for Lua packages is relative to the executable. So if I install moon.exe to d:\utils\bin, then Lua packages must go in d:\utils\bin\lua. To install Penlight simply involves copying its pl directory to this lua directory. For other systems, MoonScript shares the usual global Lua module path.

To read configuration files (both Unix and INI-style) we can use pl.config

 require 'pl'

 test = [[
 # test.config
 # Read timeout in seconds
 read.timeout=10
 # Write timeout in seconds
 write.timeout=5
 #acceptable ports
 ports = 1002,1003,1004
 ]]

 t = config.read stringio.open test
 pretty.dump t

 [[ --output--
  {
     ports = {
       1002,
       1003,
       1004
     },
     write_timeout = 5,
     read_timeout = 10
   }
 ]]

Classes

By the time people get to Lua (or Go) they have been sufficiently indoctrinated to instantly miss classes. Lua prefers to define more general metaprogramming techniques, which allows a competent user to roll any kind of custom OOP framework. (Unfortunately, too many people do this, so interoperability becomes a problem.) But ultimately all that matters is that the duck quacks. Lua does require you to indicate that the quacking applies to a particular duck, or to all ducks, much as a C++ programmer uses -> and ::.

In Lua, a method call is:

 duck:quack()

And in Moonscript:

 duck\quack()

(Idiomatic MoonScript would actually be duck\quack! but let's change one variable at a time.)

I'm not 100% crazy about the backslash here, but : has now got a more conventional job defining value/key pairs in MoonScript (and there was no other punctionation available ;))

So here is a MoonScript class, bit by bit:

 import insert,concat,remove from table

the import statement is one of those nice-to-haves which would be very useful in Lua, since the equivalent is the noisy and redundant:

 local insert,concat,remove = table.insert, table.concat, table.remove

A class is an intented block containing methods

 class List
     new: (t) =>
         @ls = t or {}

Note the colon, instead of =; this reminds us that we are putting these functions into a table. The fat arrow => means that the function has an implicit self argument; here it's equivalent to (self,t) ->.

@ls is just sugar for self.ls, as in Ruby. (The new method will be indirectly called as a constructor).

Our chosen representation for a list is a table field ls. This can be manipulated by the usual Lua table functions, which we previously imported:

 add: (item) =>
     insert @ls,item

 insert: (idx,item) =>
     insert @ls,idx,item

 remove: (idx) => remove @ls,idx

 len: => #@ls

 __tostring: => '['..(concat @ls,',')..']'

table.insert is basically two functions wrapped up as one - it both appends and inserts. #@ls is one of those collisions of punctuation that one just has to get used to in any language - it helps to read it as #self.ls (and you can use self explicitly if you like.)

Any Lua metamethod can be specified in a class. A MoonScript class is the metatable for all its instance objects, with __index pointing back at the metatable so the object can find its methods. This is the most common way of implementing 'classes' in Lua.

Unfortunately, # cannot be overloaded in Lua 5.1 for tables; __len only applies to userdata defined by C extensions. (This is one of the welcome Lua 5.2 improvements)

 find: (item) =>
     for i = 1,#@ls
         if @ls[i] == item then return i

 remove_value: (item) =>
     idx = self\find item
     self\remove idx if idx

 remove_values: (items) =>
     for item in *items do self\remove_value item

if statements come in two flavours: multiline, where the following block is indented, and single statements that use then. The same applies to for, where the single statement version has an explicit do.

Any statement/expression can also have an if clause, called a 'line decorator'.

Now list comprehensions get their turn:

 index_by: (indexes) =>
     List [@ls[idx] for idx in *indexes]

 copy: => List [v for v in *@ls]

And finally we define extension, iteration and concatenation:

 extend: (list) =>
     other = if list.__class == List then list.ls else list
     for v in *other do self\add v
     self

 __concat: (l1,l2) -> l1\copy!\extend l2

 iter: =>
     i,t,n = 0,@ls,#@ls
     ->
         i += 1
         if i <= n then t[i]

Any instance of a MoonScript class will have a __class field; I make this check because it's useful to allow a list to be extended by a plain vanilla table.

The iter method needs some explanation: in Lua, the for in statement expects a function that it can repeatedly call to get each successive value of the sequence, until that function returns nil. This method returns a closure with upvalues i, t and n.

And, exercising List:

 ls = List!

 ls\add 10
 ls\add 20

 ls\remove_value 10
 ls\remove_value 15

 ls\add 30

 ls\remove 1

 for v in *{1,2,3} do ls\add v

 print ls, ls\len!

 print ls\index_by {2,3}

 for v in ls\iter!
     print v

 print (List{'a','b'})..(List{'A','B'})

No type is complete without a definition of equality. == in MoonScript, as in Lua, compares simple values, not referencews. Objects are equal if they are the same object; (string equality works as expected because all Lua strings are guaranteed to be internally unique.) Generally, this is a good idea (none of JavaScript's 'helpfullness' here!). The relavant metamethod is __eq:

 __eq: (l1,l2) ->
     return false if l1\len! != l2\len!
     l1, l2 = l1.ls, l2.ls
     for i = 1,#l1
         return false if l1[i] != l2[i]
     return true

As another example of a Lua wish-item, note that ls\add without arguments is a valid expression, called a 'function stub',

 ls = List {1,2}
 add = ls\add
 add 3
 add 4
 assert ls == List {1,2,3,4}

ls\add 3 is short for ls.add ls 3; ls\add is a partial function application that binds the self argument.

See here for the complete code listing.

use as a DSL

One of the things I look for in new languages is how amenable their syntax is to the creation of embedded domain-specific languages.

The MoonScript author has a static website generator called sitegen where you specify your configuration with a site.moon file:

 require "sitegen"
 site = sitegen.create_site =>
   deploy_to "leaf@leafo.net", "www/sitegen"

 @title = "Sitegen"
 @url = "http://leafo.net/sitegen/"

 add "index.html"
 add "doc/ref.md"

 site\write!

We've met the fat arrow (=>) before in methods, but it can be used anywhere. Here => is shorthand for (self) ->. Now the @title shortcut makes sense, since it's just self.title.

The Lua equivalent is certainly more verbose, although I'm not sure we're losing much on readability:

 require "sitegen"
 local site = sitegen.create_site(function(self)
    deploy_to("leaf@leafo.net","www/sitegen")

 self.title = "sitegen"
 self.url = "http://leafo.net/sitegen/"

 add "index.html"
 add "doc/ref.md"
end)

 site:write()

sitegen is the best existing example of real working MoonScript. As the author says modestly, "apart from the compiler". (That's right, MoonScript is now self-hosting.)

Problems and Irritations

You'll notice the necessary parentheses when concatenating two List expressions. Although we are encouraged to leave them out unless they are necessary, the rules are not as straightforward as they could be. For instance, this equivalence contradicts the above rule; note how whitespace matters:

 L"frodo" + L"bilbo" == L("frodo") == L("bilbo")
 L "frodo" + L "bilbo" = L("frodo" + L("bilbo"))

This comes from a Lua convention that parens are not needed if the function has a single string argument, but results in a special case that is likely to bite more than one person. It's easy enough to learn to read indentation as syntax, but making a single space syntactically significant is going too far. I would see this as destructive interference from the base language.

There are other inconsistencies, like you cannot say @\f to call another method when inside a method body. (This would lead to serious punctuation clusters like @\f! so perhaps this is a feature ;))

Generally, the compiler is not so very helpful and gives non-specific parse errors, and the line it does complain about is often not the problem line anyway. So I found myself having to read the manual carefully to work out issues. It reminded me of my Seventies experience with mainframe Fortran, where the result of painstaking work at the card puncher would lead to the equivalent of 'Que?'. And run-time errors would usually end up as a six-digit error code which you would then have to look up in the IBM manuals, which were bolted down like Medieval manuscripts. We've come a long way, and it shows how important good diagnostics are to learning a system: the compiler as tutor.

But hey, it's a young language, these are all fixable faults. It's a must-play for anybody interested in the future evolution of Lua, and could find an audience among CoffeeScript enthusiasts who might appreciate a lighter, faster runtime, especially with more generally useful batteries.

Ten Programming Languages

This is a personal list of programming languages that influenced me. It isn't a representative survey, nor is it complete (being something of a programming language slut I've tried everything, once). There are several common threads in this discussion. One is that I've always enjoyed interactive prompts; there is nothing better for exploring a language or a new library. (And any language can be made interactive.) The other is that I'm allergic to long build times, which probably indicates borderline ADD or at least a lack of rigour.

Another theme is over-specialization, like FORTRAN, or paradigm-driven design, like Java. Such languages find it hard to move with the times and can end up in evolutionary cul-de-sacs.

One language-nut thing I never did was design a new language. No doubt very entertaining, but it feels like adding a new brick to the Tower of Babel. The hardest part of writing a program is to make it clear to other humans, since computers aren't fussy once you feed them correct syntax. Every good programmer needs to be a polyglot and pick the right language for the job; there is no one programming language that stretches comfortably across all domains.

FORTRAN

This is the grand daddy, and I can even give you a pop culture reference.

This was the language we were taught for first year CS at Wits University, Johannesburg. In fact, it was its last year before Pascal became the teaching language. but FORTRAN remained in my life as a scientific programmer for many years. It's widely considered a relic of the computing Stone Age, a dinosaur, although dinosaurs were sophisticated animals that dominated the world for many millions of years, and are still represented in their modern form as birds. So the clunky FORTRAN IV of my youth became Fortran 90. The old reptile is still used for high-performance computing.

This is the style of code which got people onto the Moon. The fixed format comes straight from the limitations of punch cards, with code having to start in column seven and end at column 72.

 F = FLOW
 IF (F .GE. FHIGH) GOTO 2
 C = 5.0 * (F - 32) / 9.0
 WRITE(6,"2F8.3"),F,C
 F = F + FSTEP
 GOTO 1
 CONTINUE

It has evolved into a modern-looking language (programmers have since learnt not to shout).

 real f,low,high
 low = 0
 high = 100
 step = 10
 f = low
 do while ( f <= high )
    c = 5.0 * (f - 32) / 9.0
    write(*,fmt="(2F8.3)") f,c
    f = f + step
 end do

Lots of old dinosaur code is still found in the wild, however! Trying to discover what it does in order to reimplement it is a challenging exercise, which is why keeping algorithms only in code is a bad idea, particularly if the code is written in Olde Fortran by scientists and engineers.

It remains popular, because many scientists still learn it at their mother's knee and it's a simpler language than C (despite the best efforts of the Fortran 90 committee.) This simplicity, plus built-in array operations, makes it easier for the compiler to optimize common numerical operations. And there's always the inertia of the numerical community and the availability of so many libraries in Fortran.

Pascal

Judy Bishop's second-year course in Pascal helped me to recover from Fortran and taught me structured programming.

Nicklaus Wirth's book was probably my first intellectual experience in the field. Pascal was always intended to be a teaching language, and Modula was supposed to be the version for grown-ups. Classic Pascal was a (deliberately) restricted language, and Brian Kerighan famously wrote Why Pascal is Not My Favourite Programming Language. But then you must remember that compared to FORTRAN IV it was like awakening from the primal ooze.

I remember reading somewhere (probably from the LOGO experiments) that when kids are exposed to a programming environment, they do not spontaneously structure their programs. They readily grasp the idea of giving an action a name, and then repeating it, but do not refactor complicated actions into a series of simpler actions. Such things have to be taught, and this was the single biggest thing I learnt about programming at university. After all, students have to be taught to write, since organizing language in the form needed to communicate effectively does not emerge from their native spoken language skills.

Subsequent paradigm shifts in programming education have happened, like object-orientation and the move to Java, but I can't help feeling that basic structuring skills are neglected when students' minds are immediately poured into classes. OOP downplays functions, but they are important building blocks and people need to know how to work with them. Perhaps the mental development of programming students should follow historical development more closely?

In the late Eighties, Borland sprung on the scene with Turbo Pascal, which was a marvelous, fast and practical implementation. This was my workhorse until I discovered C, but later I used Delphi for Windows GUI development and still consider it to be one of the best options available.

One of Borland Pascal's distinctive features was a very smart linker. The equivalent of object files had all the necessary type information, so that compiling and linking programs was fast even on modest hardware.

LISP

The only interactive language on the Wits mainframe was LISP, which started a lifelong love of conversational programming languages and introduced to me the notion that programs could manipulate symbols. (And by extension, manipulate programs.)

I found an old copy of John McCarthy's LISP 1.5 manual in the library, and my head was seriously opened. Recursion is a beautiful thing, and I first saw it in LISP.

Unlike others, I didn't pick up LISP later, but it planted seeds that flowered when I used Lua much later.

C

My first C experience resulted in a crashed machine, which happens when you are spoilt by Pascal's run-time array bounds checking. C is the ultimate adult programming language, and does not get in your way if you insist on shooting yourself in the foot. Again, Turbo C was the entry drug and I've been hooked ever since; I could replace my mixture of Pascal and x86 Assembly with one language.

C is not a particularly good language for GUI development. I've used it for both Windows and GTK+ development, and it gets tedious (and remains unforgiving.)

A characteristic feature of C is that the language itself is very small; everything is done with libraries. With earlier languages, doing I/O was built in; Fortran's write and Pascals writeln are part of the language and are statements; printf is a library function. So C is the language of choice for the embedded world, where programmers still worry about getting a program into 4K.

The joy of C is knowing pretty much exactly what each bit of code does, without worrying about hidden magic or operator overloading.

C++

As a meta-observation, none of the languages in this list are considered particularly cool or state-of-the-art, except LISP which has continuously evolved since 1962. This is because they are pragmatic tools for getting things done. In this respect, C++ gets a lot of respect, because it raises C to a higher level, while keeping the abstraction penality as low as possible. Learning the language was challenging (using Stanley Lippman's book) but very rewarding. Building GUIs is a good fit to object-oriented design and I went through the usual rite of passage, which was creating a minimal Windows API binding.

A fair amount of my work was maintaining C++ MFC systems. Microsoft's MFC framework is widely considered one of the worst class libraries in the known universe, and for many years the word 'Visual' in 'Visual Studio' was a cruel joke. One of its many sins was encouraging storing data as persistent object binary archives; this breaks any attempt at easy continuous evolution of a program's structure by tying it directly to the structure of the persistent store.

However, bad programs and frameworks can happen to good languages, and I developed an increasingly obsessive relationship with C++. The core idea of the obsession was that people learn best by conversational interaction with a programming language, and led to the development of UnderC and a book, C++ by Example organized around this principle. This gave me the interactive prompt that I had been missing, and that I hypothesized that students were missing.

The other idea is that a good deal of the awkwardness of big C++ development comes from an antiquated build model inherited from C. C++ compilers aren't actually slow, but they have to make a heroic effort to cope with enormous header files. With an interpreter you can keep a program alive, and only recompile the bits that need to change, reducing the need for frequent rebuild-the-world steps and the necessity to get back to the program state you need to test. Furthermore, with an interpreter a language like C++ can serve many of the roles occupied by 'scripting languages', so that there is no need for Osterhout's Dichtomy. (There are big assumptions running around wild in this paragraph, and they should be subject of a more specialized article.)

Getting the interpreter to parse the full STL was my Waterloo, and I subsequently developed a dislike for the language and its sprawling complexity. In truth it was too big a job for one primary developer, and some UnderC design deficiencies proved too great an obstacle to progress. In particular, I was not storing an internal AST representation of templates, but rather keeping their bodies as text. Blame that on my youthful decision to do physics rather than CS, I suppose.

UnderC remains bound to x86 by some assembler shenanigans, and a logical next step would be to use libffi to get more platform independence, at least for linking to C. The grand vision that it could consume any C++ shared library was briefly viable, but the continuous changes in the C++ ABI and name-mangling schemes makes that an endless Red Queen race.

I became increasingly out of step with the direction that modern C++ was taking, with growing use of templates leading to a language that was increasingly slow to compile, and often generating some of the most obscure error messages ever presented to unfortunate users.

C++ remains the hardcore choice for many applications (although I suspect that often it's premature optimization) and seems to have a long life ahead of it. I may even forgive it enough to give it another chance one day.

C#

C# is an interesting language, because it is the direct result of a commercial dispute with religious undertones. Microsoft had brought over Anders Heljsberg (the chief architect of Delphi) from Borland, to work on their Visual J++ product. This dialect implemented delegates, which are method pointers similar to Delphi's events. This was not only heretical, but against the Java licensing conditions, and for this and more egregious sins Sun won a lawsuit against Microsoft.

By this time, Microsoft had the Java bug, but to keep moving in the direction they wanted to go they needed to fork the language, and C# was born.

C# felt like a natural fit to GUI programming for me since I knew Delphi well, and anybody who has used both the Delphi VCL and Windows.System.Forms will know the family resemblance.

Unlike its estranged cousin Java, C# has kept evolving and has never been particularly afraid of breaking binary compatibility. Its model of genericity is efficient for native types (unlike collections in Java which can only contain boxed primitives), it has type inference and anonymous functions.

A consequence of this constant evolution is that .NET people are not seeking 'a better C#' in the same way that programmers in the JVM ecosystem are.

The flip side is that the .NET world has always been hooked into Microsoft's own ADD and need to sell new tools, with old acronyms dying like flies and the constant need to chase new novelties.

All in all, a great girl. But a pity about the family, as they say.

Boo

Boo isn't a well-known language, but has some interesting features. Syntactically rather like Python, except it is a statically-typed compiled .NET language with aggressive use of type inference. It also has support for compile-time metaprogramming and so can do 'smart macros'.

I did a Scintilla-based editor in Boo, and learnt its strengths and weaknesses well (Writing editors is a common hobby among programmers, since we have such a personal relationship with our tools.)

Ultimately the disillusioning realization was that the extra expressiveness of the language did not particuarly bring much advantage over using C# directly. To do Sciboo in C# would have required maybe five times as much code, but that extra code would have compiled a good deal faster than the equivalent Boo code and would have had come with much better tool support.

Writing a program in a new programming language is taking a bet on an outsider, and Sciboo would probably have caught more traction if it had been done in C#.

Still, Rodrigo "Bamboo" de Oliveira's Boo remains the second-greatest programming language to come out of Brazil.

Java

The Java moment for programmers is when they feel the advantages of a simplified C++ with an extensive library and GUI toolkit, without the need to worry about memory management and having rich run-time type information available. I had already had this epiphany with C#, but increasingly appreciated the cross-platform JVM and the excellent tooling provided by Eclipse. Ultimately, it's the ecosystem that matters, the language, libraries and tools.

Java is not a fashionable language, for various reasons. It was deliberately designed as a restricted language built around object-oriented principles, a purified C++, much as Pascal was a simplification of earlier big languages like Algol 68. A programming language built around a particular programming paradigm is like an overspecialized species in a particular ecological niche. The big dinosaurs flourished in a different climate, hotter, wetter and with higher oxygen levels. The little rats running around their feet adapted to the changed conditions and eventually conquered the world. Together with its dogmatic design (the fundamental unit was always the class) came an attitude that the designers knew best, and that is fundamentally off-putting to hackers. Modern (or at least more recently mainstream) ideas like functional programming are inherently hard to implement on such a platform.

Some of Scala's woes come from the fact that a simple little anonymous function has to be implemented as a separate class, and classes don't come cheap on the JVM; even the tiniest classes take up about half a kilobyte, mostly metadata. (The slowness of scalac means that I personally would be unlikely to enjoy Scala, despite its attractive features, as I concluded with Boo.)

If one works within the limitations, exploits its dynamic features, and ignores the endless moralizing and pontification, Java can still be a useful and efficient solution to many problems. 'Expressiveness' is not a virtue that can be used in isolation.

Lua

Lua and LISP are the only dynamic languages in this list. Of the dynamic languages, Lua has the smallest implementation and the fastest performance. Lua was designed to be easily embeddable in a host C/C++ program, and often used in big commercial games. The language implementation is about 160K, and is about two times faster than Python. LuaJIT is a heavily optimized JIT implementation that can often give compiled C a good run, which is worth bringing up when people repeat the old assertion that statically typed languages have an inherent performance advantage.

I'm not a natural candidate for learning a 'gaming' scripting language (since computers are too interesting without needing games) but I learnt Lua scripting for the SciTE editor, and naturally moved onto the desktop.

In many ways, Lua is the C of dynamic languages: lean, fast and without batteries. The team at PUC-Rio do the core language, and the community provides the batteries, much as Linus Torvalds does the Linux kernel and GNU, etc provide the userland. The analogy is not exact, since core Lua provides math, io and a very capable string library. Lua is based on the abstract platform defined by C89, and so if you need things like directory operations or sockets you need extensions outside the core. Lua Distributions are available that provide similar experience to the 'big' scripting languages, but they are not 'canonical' or 'blessed'.

One reason that Lua is used as an extension language is that the syntax is conventional and unthreatening. I often come across engineers or scientists who just cannot read curly-brace C-like languages, and they naturally start counting at one.

A common criticism from people used to Python or Ruby is that there is no actual concept of 'class' in the language. Lua provides more general metaprogramming mechanisms that makes it easy to implement a 'class' system in a few lines of code. There are in fact a number of distinct (and imcompatible) ways to do OOP, which does lead to interoperability issues.

But trying to shoehorn everything into an existing concept like 'classes' is a sign of not 'getting' a language. In Lua functions are first-class citizens and are proper closures, making a more functional style possible. Lua's single data structure is the table, which is an associative array; like JavaScript, m["key"]==m.key so that you can use tables like structs.

LuaJIT is a concrete proof that serious performance and dynamic typing are not orthogonal. However, the main challenge for dynamic languages is what used to be called 'programming-in-the-large'; how to compose large systems from subsystems and reason about them. It's easy to produce mad unmaintainable messes in dynamic languages.

It's obvious that dynamic function signatures contain less information than their static equivalent: sqr(x) is less self-documenting than sqr(double x). So for dynamic languages, there is need to document the actual types so that users of a module can use it effectively. The compiler never reads comments, of course, and no compiler in the world can make useful comments mandatory. It's a matter of discipline and having structured ways to express types as documentation.

Having explicit types also allows tools like IDEs to provide intelligent browsing and code completion. A surprising amount of information can be extracted by static analyis (for instance, David Manura's LuaInspect) but you still need in addition a machine-readable way to provide explicit type annotations.

A very interesting generalization of Lua is Fabien Fleutot's Metalua, which is an implementation which allows language extensions to be written in Metalua. For instance, this is a Metalua module which allows the following code:

 @lua
 --- normalized length of foo string.
 function len(s :: string) :: number
 ...
 end

This inserts type assertions into the code (although this can be disabled). This is reminiscent of the direction taken by Google Dart.

My own modest approach with LDoc involves extended JavaDoc-style tags:

 @lua
 --- normalized length of foo string.
 -- @tparam string s the foo string
 -- @treturn number length
 function len(s)
 ...
 end

Go

Go is modern, bold attempt to build a better C. It is garbage-collected, but compiles to native standalone executables, and has built-in strings, maps and CSP-style concurrency organized around 'goroutines' and channels. Go functions may return multiple values, like Lua. Type inference means that code outside function and struct declarations rarely needs explicit types.

Such boldness is unlikely to be universally popular, of course. For diehard C fans, 'a better C' is a nonsensical idea. For Java/C++ fans, the lack of classic OOP is a problem. The advocates of Go consider this lack to be a strength; polymorphism is supported by interfaces (as in Java) but there's no inheritance in the usual sense; you can 'borrow' implementation from a type using embedding. Inheritance has its own issues, after all, such as the Fragile Base Class problem, where a class is at the mercy of its subclasses (or vice versa, depending on the interpretation.)

Likewise, classic exception handling is replaced by handle-errors-at-source and panicking (with optional restore) when that is not feasible.

It's natural to miss the comforts of home, but learning a new language often involves unlearning the mental habits of an earlier language.

Go feels like a dynamic language, not only in time getting from Save to Run, but by the lack of explicit typing. Interfaces work as a user of a dynamic language would expect: if a function expects an interface that only contains a Read() []string method, then any object that provides such a method is acceptable. It is rather like compile-time duck-typing, and eases the interdependency problem that plagues languages like Java. But these things are all vigorously checked (Go does not do implicit typecasts); typos tend not to blow up prograns at run-time.

The language fits easily in the head. Goroutines+channels are easier and less problematic than traditional threading models. Go is clearly designed to do serious work on the server, and the standard library provides good support for networking and protocols, so that's where most of the adoption will take place.

A criticism from the C++ perspective is that too much functionality is built-in, and the language lacks the abstraction mechanism to further extend itself. For instance, Go's maps cannot be defined using Go itself. But making maps a primitve type remains a good pragmatic decision for Go, because std::map and friends in C++ impacts on end users in terms of the large amount of template headers needed to be parsed and the resulting unhelpful errors if anything does wrong.

I've personally used it for processing binary data files where I would otherwise use C, and it feels productive in this role. I miss the standard C promotion rules, so that an int will not be promoted to a float64 in the right context - this means a lot of explicit typecasting. Also, 'variable not used' errors are irritating when prototyping. But mostly I'm enjoying myself, and learning a new language. Which ultimately is what this article is all about.