Wednesday, 2 October 2013

C++: Some Consequences of a Design Decision

Top Dog

C++ has a very solid position as the programming language which makes the least performance compromises while providing good abstraction mechanisms that allow code to be written at a high level. In many ways, it's an easier and safer language to learn than C, if you stick to the same imperative style but use std::string and the provided containers.

Both of these statements are of course open to debate. The first statement is true, if we look at the usage of this language in performance-critical applications. The second is often challenged. To quote Andrei Alexandrescu's comment in this Reddit comment:

In my opinion C++ is a language for experts and experts only [...] It has enough pitfalls in virtually all of its core constructs to make its use by benevolent amateurs virtually impossible except for the most trivial applications.

Ouch! That's fair enough; we were comparing it to C anyway (which is definitely not for sissies). It is not really a programming language for civilians, and not a good first language for anyone other than a would-be professional. (In fact I'd say you would get a better all-round education in C, even if later you turn in relief to non-military languages; learning C++ is mostly good for ... programming in C++.)

Error, Error on the -Wall

The big hurdle is the first one, and that's making sense of the error messages:

 #include <iostream>
 #include <string>
 #include <list>
 using namespace std; // so sue me
 int main()
     list<string> ls;
     string s = "hello";
     ls.append(" world");
     cout << ls << endl;
     return 0;

This isn't a bad attempt at a C++ program at all, leaving aside the pedantic belief that using namespace std is bad. (I take the pragmatic view that anybody is free to inject whatever namespaces they care to within their files, and not take away this freedom from others by injecting namespaces within header files. C++ is very good at resolving ambiguous name references and everyone should know the contents of std anyway.)

We get nearly four hundred lines of error messages, full of implementation details. In this case, the abstractions are leaking all over the user!

Verity Stob once suggested that the thing to do was to write a Perl script to parse the error output. This was very funny and true, but using Perl would increase the number of problems. My practical way of realising Verity's joke is to use lake and a suitable plugin:

 C:\Users\steve\dev\dev>lake -L filter.cpp error.cpp
 g++ -c -O2 -Wall -MMD  error.cpp -o error.o
 error.cpp: In function 'int main()':
 error.cpp:10:8: error: 'list<string >' has no member named 'append'
      ls.append(" world");
 error.cpp:11:10: error: no match for 'operator<<' (operand types are 'ostream {a
 ka ostream}' and 'list<string >')
      cout << ls << endl;
 lake: failed with code 1

Now our noob has a fighting chance, and can now go to the reference and actually find the appropriate method.

Templates Considered Harmful

The real issue is that the C++ standard libraries over-use generics. std::string and std::stream could be plain classes, as they once were. At this point, there will be someone suggesting that I am a plain ASCII bigot and forgetting the need for wstring and so forth. Fine, let them be plain classes as well. An incredible amount of ingenuity went into making templated string types work, and the library designers could have made their life easier by using a low-tech solution. Generally, we should not pander to library designers and their desires, since they chose the hard road: their job is to use the right level of abstraction and not complicate things unnecessarily.

C++'s standard generic containers are fantastically useful, but their design is overcomplicated by being also parameterized by an allocator. This is a useful feature for those that need it, but there could be two versions of (say) std::list overloaded by template parameters, which can be done in C++11 with variadic templates. This makes life a bit harder for library implementers, but they are precisely the people who can manage complexity better than users.

The Standard is the Standard, no point in moaning. But let's do an experiment to see what the consequences of a simplified standard library. I emphasize that tinycpp is an experiment, not a proposal (modest or otherwise). It originally was done for the UnderC project, since the interpreter could not cope with the full STL headers, and I've since filled in a few gaps. Here it's purpose is allow some numbers to be generated, since qualitative opinion is all too common.

These simplified 'fake' classes directly give us better error messages, especially if the compile bombs out on the first error. (Often after the first error the compiler is merely sharing its confusion.)

 $ g++ -Wfatal-errors -Itiny error.cpp tiny/iostream.o tiny/string.o
 error.cpp: In function 'int main()':
 error.cpp:10:8: error: 'class std::list<string>' has no member named 'append'
      ls.append(" world");
 compilation terminated due to -Wfatal-errors.

It's easy to forget the initial difficulty of learning to ride the bicycle, and to scorn training wheels as a useful means to that end.

Templates Slow you Down

People say 'C++ compiles slowly' but this not really true. A little C++ program will involve in about 20Kloc of headers being processed, a lot of that being template code. Using the tinycpp library that goes down to 1.4Kloc.

The three compilers tested here are mingw 4.8 on Windows 7 64-bit, MSVC 2010 on the same machine, and gcc 4.6 in a Linux Mint 32-bit VM.

Here is a comparison of build times for standard vs tinycpp:

  • mingw 0.63 -> 0.33
  • gcc 0.60 -> 0.20
  • msvc 0.82 -> 0.17

As always, gcc works better on Linux, even in a VM, and it's no longer slower than MSVC. In all cases the tinycpp version compiles significantly faster.

C++ programmers can get a bit defensive about compile times, and often end up suggesting throwing hardware at the problem. There seems to be a "You're in the Marines now boy!" macho attitude that wanting to build faster is a sign of civilian weakness and poor attention span. This attitude is off-putting and gets in the way of better engineering solutions. Most people just suck it up and play with light sabres.

Templates Make you Fat

With small programs, these compilers produce small executables when they are allowed to link dynamically to the C++ library. This is not considered a problem on Linux, because obviously everyone has upgraded to the latest distro. But if you want to chase cool new C++11 features you may find that most of your users don't have the cool new libstdc++ needed to run your program.

It is (curiously enough) easier to get a new shiny GCC for Windows precisely because it's not considered part of the system. Executables rule in Windows, so it's alarming to find that a small program linked statically against libstc++ is rather large, nearly 600kb for Windows. And since libstc++ is not part of Windows you (again) have to suck it up. (And this is definitely what Alexandrescu would consider a 'trivial application'.)

You can get down to 174Kb using the fake tinycpp libraries, which suggests that an up-to-date and properly engineered version of std-tiny would be useful for delivering executables, not just for speed and noob-friendliness.

MSVC does static linking much more efficiently; the numbers are 170Kb (std) and 95Kb (tiny). The resulting executables have no C runtime dependencies whatsoever. Which suggests that MSVC is (at least) a good choice for building releases for distribution. Using a cross-platform compiler-aware tool like CMake or Lake can make that less painful. Not an ideologically comfortable recommendation to accept, true, but whatever works best. (The command-line version of MSVC 2010 is freely available.)

This preoccupation with executable sizes seems last-century by now (after all, Go users are fine with megabyte executables since they see that as the price of no other runtime dependencies.) And large executables are not slower, providing the actual code that's executing at any point is compact enough to be cache-friendly. So perhaps I'm just showing my age at this point, although please note that resource-limited devices are much more common than desktop computers.

No Free Lunches

C++ programmers like the phrase 'abstraction overhead' because C++ is very good at reducing this to zero in terms of run-time. Often this is at the cost of executable size, compile time and confusing errors. This may be an acceptable price, but it is not free.

C++ is what it is; it is unlikely to change that much, except get even slower to compile as the Boost libraries move into the Standard library. But I think that there are some lessons to be learned for new languages:

  • keep the standard library as simple as possible: library developers should not have too much fun (They should write cool applications that use their libraries instead to get excess cleverness out of their system.)
  • error messages should not burden the user with implementation details; this means that the abstraction is leaking badly.
  • compile time still matters. Perhaps the people who use C++ more regularly are more likely to be those who like to think upfront (like embedded programmers) but this is not the only cognitive style that flourishes in programming. It is a mistake to think that long build times are a necessary evil, since with C++ they largely come from an outdated compilation model. New languages can do better than that.

Monday, 2 September 2013

Nimrod: The Return of Pascal

Why learn Another Language?

The first answer is: because it's fun. Just as a botanist is excited to find a new plant, programming language nerds like trying out new languages. Secondly, any new language uses new strategies for dealing with the basic problems of communicating algorithms to computers and intents to other programmers. So it is the most sincere form of criticism: a working implementation to constrast with the approaches taken by other languages. There's far too much armchairing and bikeshedding involved in discussions about languages, and you have to admire a guy who has spent a sizeable chunk of his life trying something new like Nimrod's author, Andreas Rumpf.
If you're not a language nerd, a new language might provide a solution to an actual computing problem you are facing. (Who would have guessed?)

Hello, Nimrod

For this exercise, I'm assuming a Unix-like system, but pre-compiled installers for Nimrod on Windows are available.
First, download and build Nimrod from here. It only takes a few minutes, and after making the suggested symlink nimrod will be on your path. In that directory, you will find a most useful examples folder, and the documentation is doc/manual.html for the manual,doc/tut1.html for the tutorial,2doc/lib.html for the standard library.

Here is a slightly non-trivial Hello-world application, just to test the compiler:

 # hello.nim: Hello, World!
 var name = "World"
 echo("Hello " & name & '!')

Compiling involves the simple incantation nimrod c hello.nim, which will generate a very chatty record of the compilation, and an executable hello. This has no external dependencies apart from libc and comes at about 130Kb; with nimrod c -d:release hello.nim the compiler agressively removes unneeded code and we are down to 39Kb.

This is the first take-home about Nimrod; it compiles into native code using the available C compiler and so can take advantage of all the optimization work that's gone into beasts like GCC and MSVC. There is no special runtime, so these executables can be shared with your colleagues without fuss. In the library documention doc/lib.html, 'pure' libraries will not introduce extra dependencies. Whereas (for instance) the re regular expression library currently implies an external dependency on PCRE.
The verbosity is interesting the first few times, and thereafter becomes tedious. I've defined these bash aliases to get cleaner output:

 $ alias nc='nimrod c --verbosity:0'
 $ alias ncr='nimrod c -d:release --verbosity:0'

Training a programmer's editor to present Nimrod code nicely is not difficult; using Python highlighting works well since the languages share many keywords. The main thing to remember is that Nimrod does not like tabs (thank the Gods!) here are some SciTE property definitions which you can put into your user configuration file (Options|Open User Options File); now F5 means 'compile if needed and run' and F7 just means 'compile'.

After a few invocations to get all the tools in memory, this compilation takes less than 200ms on this rather elderly machine. So the second take-home is that the compiler is fast (although not as fast as Go) and definitely faster than C++ or Scala. In particular, syntax errors will be detected very quickly.

A First Look

This code looks very much like a typical 'scripting' language, with hash-comments, explicitly-declared variables and string operations like concatenation (&). (A separate concatenation operator is a good decision, by the way; Javascript has a number of famous ambiguities that come from + meaning two very different things.)

However, this is not dynamic typing:

 # types.nim
 var s = "hello"
 var i = 10
 s = i
 $ nc types
 examples/types.nim(4, 5) Error: type mismatch: got (int) but expected 'string'

So s is statically-typed as 'string', i is typed as 'int', and no sane conversion should ever make an integer into a string implicitly. Nimrod does local type inference which examines the expression on the right-hand side and uses that type for the declared variable, just like the same statement would do in Go. Another good thing, since a variable cannot change type underneath you and you really need as many errors to happen at compile-time. The resulting code is also much more efficient than dynamically-typed code.

The next program looks very much like Python:

 # args.nim
 import os
 for i in 0..ParamCount():
 $ nc args
 Hint: operation successful (14123 lines compiled; 0.374 sec total; 12.122MB) [SuccessX]
 $ ./args one two three

But beware of surface resemblences; sharks and orcas look much the same, but are very different animals. The language that Nimrod reminds me of here is Rodrigo 'Bamboo' de Oliveira's Boo, the second-greatest programming language to come from Brazil. His comment is "We also love the Monty Python TV show! - but Boo is not Python". So Pythonistas should not assume that they can automatically skip the first semester with Nimrod. The first difference to note is that import brings all functions from the module into the current scope.

Apart from basic syntax, built-in functions like len and repr work mostly as you would expect from Python. Slicing is supported, but note the different syntax:

 var S = "hello dolly"
 var A = [10,20,30,40]
 var B = A[1..2]
 echo(len(A)," ",len(B)," ",len(S))
 for x in B: echo(x)
 # --->
 4 2 11
 [10, 20, 30, 40]

Type inference is fine and dandy, but is not letting us have the full picture. The 'lists' in square brackets are arrays, and they are fixed size.

The Return of Pascal

To a first approximation, an orca is a wolf in shark's clothing. Simularly, the language that Nimrod most matches in nature is Pascal:

 # pascal.nim
     TChars = range['A'..'C']
     TCharArray = array[TChars,int]
 var ch: TCharArray
 for c in 'A'..'C':
     ch[c] = ord(c)
 for c in low(ch)..high(ch):
     echo("char ",c,' ',ch[c])
 # --->
 char A 65
 char B 66
 char C 67

Paws have become flippers (= instead of :=, no semicolons or begin..end blocks) but this is classic Pascal typing, with subranges and array types declared over arbitrary ordinal types. So accessing ch['Z'] is a compile error 'index out of bounds'. Also, 'Z' is of type char and "Z" is of type string - quite distinct as they are in C as well. Like Pascal, arrays are always bounds checked, but this can be disabled by pragmas. The T convention for naming types should be familiar with anyone who was a Borland fan.

Please note that variables are case-insensitive! Underscores are ignored as well. (This may well change.)

Another indication that Nimrod comes from the Niklas Wirth school is that functions are called procedures, whether they return something or not.

 # proc.nim
 proc sqr(x: float): float = x*x
 # -->

You should not assume that float means 32-bits; the manual says "the compiler chooses the processor's fastest floating point type" and this usually is float64; there is also float32 if you wish to be explicit, just as in Go. (The usual conversions between integers and floats are allowed, since they are widening.) In a similar way, int always has the size of a pointer on the system (which is not true for C), and there is intXX where XX is 8,16,32 or 64.
Also as with Pascal, arguments may be passed by reference:

 # var.nim
 proc modifies (i: var int) =
     i += 1
 var i = 10
 for k in 1..3:
 # --->

This is a procedure that returns nothing. Every language draws a line in the sand somewhere and says "I don't think you should do that, Dave". One of Nimrod's rules is that you cannot just discard the results of a function that returns a value, unless you use the keyword discard before it like discard fun(), rather as we say (void)fun(); in C.

There is fairly standard exception handling. A cool novelty is that finally or except can be used as standalone statements:

 proc throws(msg: string) =
     raise newException(E_base, msg)
 proc blows() =
     finally:   echo "got it!"
     echo "pre"
     throws("blew up!")
     echo "post"
 # --->
 got it!
 Traceback (most recent call last)
 finally.nim(10)          finally
 finally.nim(7)           blows
 finally.nim(2)           throws
 Error: unhandled exception: blew up! [E_Base]

This is very similar in effect to Go's defer mechanism, and allows for deterministic cleanup.

Tuples are Structs

It's often better to take the Python strategy and return multiple results using a tuple

 # tuple.nim
     TPerson = tuple [name: string, age:int]
 proc result(): TPerson = ("Bob",42)
 var r = result()
 echo(r)             # default stringification
 echo (       # access by field name
 var (name,age) = r  # tuple unpacking
 echo (name,"|",age)
 # --->
 (name: Bob, age: 42)

Different tuple-types are equivalent if they have the same fields and types in order ('structural equivalence'). Nimrod tuples are mutable, and you should think of them more as akin to C's struct.
Functions defined over a type have a most curious and interesting property. Contining with tuple.nim we write a silly accessor function:

 proc name_of(t: TPerson): string =
 # --->

That last line is something to think about: we've got something like 'methods' by just using the function after the dot, as if it were a field; in fact you typically leave off the () in this case and have something very much like a read-only property.

'List' was a Bad Name Anyway...

I mentioned that [10,20] is a fixed-size array, which is the most efficient representation. Sequences can be extended, like C++'s vector or Python's List types.

 # Using seq constructor and append elements
 var ss = @["one","two"]
 # using newSeq, allocate up front
 var strings : seq[string]
 newSeq(strings, 3)
 strings[0] = "The fourth"
 strings[1] = "assignment"
 strings[2] = "would crash"
 #strings[3] = "out of bounds"

Using sequences of strings and the parseopt module, here is a simple implementation of the BSD head utility. The release executable is 58Kb, which is an order of magnitude smaller than the equivalent Go stripped executable. It's only 54 lines, but a little big to be an inline example. The case statement is very Pascal-like:

 case kind
 of cmdArgument:
 of cmdLongOption, cmdShortOption:
   case key
   of "help", "h": showUsage(nil)
   of "n":
     n = parseInt(val)
   of "version", "v":

parseopt isn't fully GNU compatible: in particular, you have to say ./head -n=3 head.nim rather than -n3 or -n 3. The code style is a bit low-level for my taste; compare with lapp; a well-behaved command-line tool must always provide its usage, so why not reuse that text to describe flags, arguments and their types? This style works particularly well with dynamic languages, but it can be done with Nimrod. Here is head, revised:

 # head.nim
 import lapp
 let args = parse"""
 head [flags] filename
   -n: (default 10) number of lines
   -v,--version: version
   <files> (default stdin...)
     n = args["n"].asInt
     files = args["files"].asSeq
 proc head(f: TFile, n: int) =
     var i = 0
     for line in f.lines:
         i += 1
         if i == n: break
 if len(files) == 1:
     for f in files:
         echo("----- ",f.fileName)

Associative arrays are the key here, plus a variant value type. lapp ensures that numerical flags are correctly converted, files are opened (and afterwards closes them on a exit hook set with addQuitProc). There are some conventions to be followed:

  • flags may have a short alias; the long name is always used to access the value
  • flags are bool values that default to false
  • parameters are enclosed in <...> and are string values with no default
  • you can specify the type explicitly: bool,int,float,string,infile',outfile, or set the default and have the type infered from that:stdinandstdout` have their usual meanings

One of the really cool things about type inference is that so many of the implementation details are hidden from users of a library. This is obviously good for the user, who has less to remember, but also for the library implementer, who has freedom to change the internal details of the implementation. It leads to a style which looks and feels like dynamic code, but is strictly typed with meaningful compile-time errors.

Here the type of args is irrelevant; it is an associative array between flag/argument names and some unspecified value type, which has known fields. (In fact, this version of lapp only exports parse, the fields, and a suitable definition of [] from 'tables')

'class' is not a transferable idea

People tend to reason from simularity, so the naive nature watcher constructs a false homomorphism between sharks and orcas. I fell into this trap, assuming 'inheritance' means 'polymorphism using virtual method tables'. Nimrod's optimization attitude is showing here: "Nimrod does not produce a virtual table, but generates dispatch trees. This avoids the expensive indirect branch for method calls and enables inlining". That's right, procedures are always statically dispatched. If you want methods, you need a different construct, multi-methods:

 # class.nim
     TPerson = object of TObject
         name: string
         age: int
 proc setPerson(p: ref TPerson, name: string, age:int) = = name
     p.age = age
 proc newPerson(name: string, age:int): ref TPerson =
 method greeting(p: ref TPerson):string = "Hello " & & ", age " & $p.age
     TGerman = object of TPerson
 proc newGerman(name: string, age:int): ref TGerman =
 method greeting(p: ref TGerman):string = "Hallo " & & ", " & $p.age & " Jahre alt"
 var bob = newPerson("Bob",32)
 var hans = newGerman("Hans",30)
 proc sayit(p: ref TPerson) = echo p.greeting
 # --->
 Hello Bob, age 32
 Hallo Hans, 30 Jahre alt

Here we are making objects which are references (by default they are value types, like tuples, unlike java), initialized with the standard procedure new. Note the Pascal-like special variable result in procedures!

As expected, you may pass Germans to sayit, because a German is a person, but greeting has to be declared as a method for this to work; if it were a proc, we would get a warning about the second greeting being unused, and Germans are then addressed in English.
The cool thing about multi-methods is that they restore symmetry; a traditional polymorphic call is only polymorphic in a. This makes sense in a language where dot method notation is just sugar for procedure calls where the first argument matches the type.

Generics Everywhere

Consider this, where no type is given for the argument of sqr:

 proc sqr (x): auto = x*x
 echo sqr(10)
 echo sqr(1.2)
 # -->
sqr is implicitly generic, and is constructed twice, first for int and then for float. Comparing a similar thing in Boo reveals a key difference:
 def sqr (x):
     return x*x

Here the type of x is duck, where Boo switches to late binding.

Both archieve the same result; sqr can be passed anything that knows how to multiply with itself, but Nimrod wants to generate the best possible code, at the cost of more code generation. The more general way of declaring generic functions goes like:

 proc sqr[T] (x: T): T = x*x

Another example of Nimrod being conservative about your memory needs would be declaring a very large array of strings. In languages where string is a value type like C++ and Go, this would contain valid strings, but in Nimrod the entries are nil until explicitly initialized. So string values can be nil (like Java) which can be a fertile source of run-time errors, but the decision on how much heap to throw at the data structure is left to you, which is a very C-like design decision. Strings in Nimrod (however) are mutable and do copy by value.

Generics make it easy to write operations over containers. Here is map with an anonymous procedure:

   a = [1, 2, 3, 4]
   b = map(a, proc(x: int): int = 2*x)
 for x in b: echo x
 # --->

Anonymous procedures are a little verbose (as they are in Go), but there is a trick. We use a template which is a higher-order generic that rewrites expressions, much like a preprocessor macro in C/C++:

 template F(T: typedesc, f:expr):expr =
     (proc(x:T):T = f)
 b = map(a, F(int, 2*x))

Nimrod achieves the power of the C preprocessor in an elegant fashion, integrated into the language itself. The when statement works with compile-time constants and only generates code for the correct condition, much like a #if chain.

 when sizeof(int) == 2:
   echo("running on a 16 bit system!")
 elif sizeof(int) == 4:
   echo("running on a 32 bit system!")
 elif sizeof(int) == 8:
   echo("running on a 64 bit system!")
   echo("cannot happen!")

LIke if, it can be used in an expression context:

 const dirsep = when hostOS == "windows": '\\' else: '/'

A clever use is to conditionally add testing code to a module when it's compiled and run as a program. These tests can be as detailed as you like, because they will not bloat the compiled library.

 # sqr.nim
 proc sqr *[T](x: T): T = x*x
 when isMainModule:  # predefined constant
     assert(sqr(10) == 100)

As you might expect by now, Nimrod does not provide run-time reflection like Java or Go because it would burden code that does not need it - again, this is C++'s "Don't Pay for what you Don't use". But there is compile-time reflection, implemented by the typeinfo module, which acts as a static equivalent of Go's reflect package.

Second Impressions

There's no doubt that finding errors as early as possible using a compiler (or some other static code analysis tool) is better than finding them later as run-time errors. In dynamic languages we are always at the mercy of a spelling mistake. But static compilation has a cost in time (build times do matter) and in complexity.

Having done about a thousand lines of working Nimrod code, I feel I can express an opinion on the language. Most code is straightforward and free of explicit type annotations, and the compiler quickly gives good error messages. Run-time errors come with a useful stack trace, and mostly come from nil references. It's commonly thought that nillable values are a big mistake (C.A.R Hoare once called it his "billion dollar mistake") but a nil string value is much better at trashing a program. And this is good - fail hard and early!

However, you do need to understand some things to interpret the error messages correctly:

 let a = [1,2,3]
 echo a
 nimex/errors.nim(2, 6) Error: type mismatch: got (Array constructor[0..2, int])
 but expected one of:
 system.$(x: TEnum): string
 system.$(x: int64): string
 system.$(x: string): string
 system.$(x: uint64): string
 system.$(x: T): string
 system.$(x: int): string
 system.$(x: char): string
 system.$(x: T): string
 system.$(x: bool): string
 system.$(x: cstring): string
 system.$(x: float): string

You have to know that echo uses the 'stringify' operator $ on its arguments - then we can interpret this error as being "I don't know how to make a string from an array". The compiler then helpfully presents all the overloaded versions of $ active in this program. Of course, this is scary to people from a dynamic background who were beguiled by Nimrod's surface 'Python-like' syntax. Coming from a C++ background, I'm prepared for this way of doing things, and know that the solution looks like this (quote operators in backticks to define them):

 proc `$`[T](a: openarray[T]): string =
     var s = ""
     for e in a:
         s.add(' ')
     return s

(Mutable strings take some getting used to). This solution will work for any arrays or sequences with elements that understand $, and is very efficient, because Nimrod iterators over sequences are zero-overhead - effectively plain loops over elements.

There is a non-trivial learning curve; a motivated polyglot can learn enough in a week to be productive, but polyglots aren't so common. A new language comes with a shortage of tutorial material/books and a small community. This means that Google is not your friend, and last I checked there were two questions on Stackoverflow, one of which concerned a Brainfuck interpreter. There does however seem to be some action on Github.

A language thrives (like any life form) when it finds a niche in which it is competitive. For Lua, that has been providing a lightweight, powerful yet accessible embeddable scripting language. It has been adopted by many game developers as a way of not writing everything in C++, which is productive in two important ways: small changes to game logic do not need expensive rebuilds and don't require restarting the game; plus lesser mortals can contribute. Professional game programmers tend not to do things simply because they are cool, and so there is a market for Lua skills.

Nimrod is a good fit where C and C++ are often used. We've seen that 'userland' utilities like head can be efficiently implemented in Nimrod, and the resulting executables are typically less than a 100kb and usually have no external dependencies. This makes it a good fit for CGI since they will load as fast as C. With Go, people found statically-linked executables a good way to manage the problem of managing dependencies on deployed machines. Nimrod provides this without Go's approach of reimplementing the whole C runtime.

But the server niche requires well-tested frameworks and libraries, which can only happen with wider adoption. Thus there is a vicious circle that any new language must face; use comes from maturity, and maturity comes from use.
It's well suited to data processing and numerical tasks; operator overloading makes standard mathematical notation possible, and generics make algorithms efficient. Here again having some choice of existing libraries would make all the difference. However, it is relatively easy to bind to C libraries (since the compiler output is C) and there is a c2nim tool for processing C headers.

A particularly cool application is for embedded systems. Here the realities are merciless; embedded processors are everywhere and need to be cheap, and you can't get much memory for pennies. As a consequence, C dominates this field, and it's nasty. I can honestly say that this is my least favourite kind of programming; the preprocessor hacks alone would make Dijkstra lie down and cry. Here Andreas describes how Nimrod can be compiled with a stripped-down library with no OS support, and compiled on a 16bit AVR processor. Nimrod is probably the only new language which has the minimal attitude and metaprogramming capability to be an effective contender in this space, which is traditionally the last bastion of C.

Garbage collection is something that's often used to separate system and application languages. It's hard to add it to an existing language, and hard to remove it from a language, since it is so damn convenient.
A kernel has to manage every byte so that the userland can afford to waste memory; game programmers hate compulsory 'stop the world' GC which tends to happen when you're doing something more important. And embedded controllers often don't even have malloc. See how Nimrod's Garbage Collector works; it is low-overhead, uses reference counting and can be switched off temporarily (unlike with the Dalvik VM on Android)

In summary, Nimrod is a very rich and powerful statically-typed language which relentlessly uses compile-time metaprogramming to achieve its goals of delivering compact and efficient programs. Whether it finds its niche is an open question, but it deserves to be given a chance, and is well worth exploring further.

Update: The title is definitely a mistake, because Pascal represented a simplification of existing practice and was intended as a teaching language.  If I had said Object Pascal then it wouldn't be so bad, since that grew into a genuinely useful language for building large systems.  But Nimrod is influenced by many other languages, so any 'Nimrod is like X'  will always be a simplfication; it is what it is.

It's been pointed out that Javascript's problem is not lack of a concatenation operator, but implicit conversions: C++ lacks the separation also but would never confuse concatenating strings with adding numbers.  There is a similar implicit conversion in Lua (one of the few warts in its design) but the operators are separate, as they should be.

Monday, 12 August 2013

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
> m = one:1, two:2
> m
> for x in *ls \
>>  print x

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

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

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()}
> if 0 then 'ok'
> if 1 > 2 then 'ok' else 'nope'
> ls = for i = 1,3 do i
> ls
> [i for i = 1,3]

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

(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

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

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

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

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

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

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:


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.