What tools are there for functional programming in C?

Adam_Rosenfield_9530 0
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I've been thinking a lot lately about how to go about doing functional programming in C (not C++). Obviously, C is a procedural language and doesn't really support functional programming natively.

Are there any compiler/language extensions that add some functional programming constructs to the language? GCC provides nested functions as a language extension; nested functions can access variables from the parent stack frame, but this is still a long way away from mature closures.

For example, one thing that I think could be really useful in C is that anywhere where a function pointer is expected, you could be able to pass a lambda expression, creating a closure which decays into a function pointer. C++0x is going to include lambda expressions (which I think is awesome); however, I'm looking for tools applicable to straight C.

[Edit] To clarify, I'm not trying to solve a particular problem in C that would be more suited to functional programming; I'm merely curious about what tools are out there if I wanted to do so.

c functional-programming
13 Answers

Functional programming is not about lambdas, it is all about pure functions. So the following broadly promote functional style:

  1. Only use function arguments, do not use global state.

  2. Minimise side effects i.e. printf, or any IO. Return data describing IO which can be executed instead of causing the side effects directly in all functions.

This can be achieved in plain c, no need for magic.

user_16738680

FFCALL lets you build closures in C -- callback = alloc_callback(&function, data) returns a function pointer such that callback(arg1, ...) is equivalent to calling function(data, arg1, ...). You will have to handle garbage collection manually, though.

Relatedly, blocks have been added to Apple's fork of GCC; they're not function pointers, but they let you pass around lambdas while avoiding the need to build and free storage for captured variables by hand (effectively, some copying and reference counting happens, hidden behind some syntactic sugar and runtime libraries).

ephemient_207130

The main thing that comes to mind is the use of code generators. Would you be willing to program in a different language that provided the functional programming and then generate the C code from that?

If that's not an attractive option, then you could abuse CPP to get part of the way there. The macro system should let you emulate some functional programming ideas. I've heard tell that gcc is implemented this way but I've never checked.

C can of course pass functions around using function pointers, the main problems are lack of closures and the type system tends to get in the way. You could explore more powerful macro systems than CPP such as M4. I guess ultimately, what I'm suggesting is that true C isn't up to the task without great effort but you could extend C to make it be up to the task. That extension would look the most like C if you use CPP or you could go to the other end of the spectrum and generate C code from some other language.

Jason_Dagit_51130

If you want to implement closures, you'll have to get groady with assembly language and stack swapping/management. Not recommending against it, just saying that's what you'll have to do.

Not sure how you'll handle anonymous functions in C. On a von Neumann machine, you could do anonymous functions in asm, though.

Vlion_262270
ja_154670

Well quite a few programming languages are written in C. And some of them support functions as first class citizens, languages in that area are ecl (embbedabble common lisp IIRC), Gnu Smalltalk (gst) (Smalltalk has blocks), then there are libraries for "closures" e.g in glib2 http://library.gnome.org/devel/gobject/unstable/chapter-signal.html#closure which at least got near functional programming. So maybe using some of those implementations to do functional programming may be an option.

Well or you can go learning Ocaml, Haskell, Mozart/Oz or the like ;-)

Regards

Friedrich_150680

What is it about C that you want to make functional, the syntax or the semantics? The semantics of functional programming could certainly be added to the C compiler, but by the time you were done, you'd essentially have the equivalent of one of the existing functional languages, such as Scheme, Haskell, etc.

It would be a better use of time to just learn the syntax of those languages which directly support those semantics.

Barry_Brown_173120
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