[Haskell-beginners] Compiling C into Haskell

[Thomas Davie]

One comment,

The question is a dumb one, and makes the standard programmer assumption that you want one mega-language to kill all other languages.

Haskell has some cases in which it's fast, quick to code, and very useful.
C also has some cases in which it's fast, quick to code, and very useful.

Use Haskell where the first holds, use C where the second holds, use something else where neither holds.

Bob

On 12 Apr 2010, at 15:10, Hein Hundal wrote:

> Hi,
> 
>   I have been playing around with Haskell for about a month now and reading the nice book "Real World Haskell."  My main reason for learning Haskell is that I want to code up some machine learning projects (heavy use of matricies).  Normally, I use Mathematica and mix it with C++, but Mathematica is proprietary, slow, and can't produce executable while C++ is verbose.  Learning Haskell has been fun; however, I have been a little worried that I will sacrifice too much performance when coding in Haskell. 
> 
>   So, I recently asked one of my friends the following question, "Say you had a C program.  Can you always convert it to Haskell in such a way that the compiled Haskell is not too slow and does not need too much memory?"  Supposing that too slow means slower than 1/4 the speed of C and too much memory means twice the memory of C.  Hopefully, someone that knows Haskell well can comment on this question.  
> 
>   I am not sure, but I think the answer is yes, such a conversion can always be done and creating a C to Haskell compiler with the above performance constraints is not extremely hard.  I started thinking about how a compiler might convert a simple C program into Haskell.  Below I will paste a C program and the compile-by-hand Haskell code.  It seems to me that the ideas I used to create the Haskell code can be implemented in a compiler that converts a simple subset of C into Haskell.  I was thinking about restricting the C to one data type 32bit-integers, arithmetic (+-*/%), assignment (=), comparison (<,>,==,<=,>=), the if-condition-codeblock construct, and the while-condition-codeblock construct.  (I would also like to do integer arrays, but I have not read about mutable arrays and monads yet.)
> 
>   Any comments?
> 
> Cheers,
> Hein H.
> 
> --------------------C to Haskell------------------------
> module Main where
> 
> -- Convert a C-Program line by line into Haskell
> --
> -- 1-- #include <stdio.h>
> -- 2-- void main()
> -- 3-- {
> -- 4--    long i,j;
> -- 5--    i=7;
> -- 6--    j=0;
> -- 7--    while(j<10000)
> -- 8--    {
> -- 9--       if ((i % 17)== 11) 
> --10--          i = i*2;
> --11--       if ((i % 35)== 12) 
> --12--          i = i+13;
> --13--       if (i> 1000) 
> --14--          i = i - 1000;
> --15--       i++;
> --16--       j++;
> --17--    }
> --18--    printf("%ld", i);
> --19--}
> 
> 
> --assignment statements
> line5  (i,j)  = (7,j)
> line6  (i,j)  = (i,0)
> line10 (i,j)  = (i*2,j)
> line12 (i,j)  = (i+13,j)
> line14 (i,j)  = (i-1000,j)
> line15 (i,j)  = (i+1,j)
> line16 (i,j)  = (i,j+1)
> 
> --while statement
> line7test (i,j) = j >=100000000
> line7 (i,j) = until line7test body9To16 (i,j)
> 
> --if statements
> line9  (i,j) = if ((mod i 17) == 11) 
>   then line10 (i,j) else (i,j)
> line11 (i,j) = if ((mod i 35) == 12) 
>   then line12 (i,j) else (i,j)
> line13 (i,j) = if (i>1000) 
>   then line14 (i,j) else (i,j)
> 
> -- code blocks
> mainprogram :: (Int, Int) -> (Int, Int)
> mainprogram = line7  . line6  . line5
> body9To16   = line16 . line15 . line13 . line11 . line9 
> 
> 
> main :: IO ()
> main = putStrLn(show(mainprogram (0,0)))
> 
> -----------------end of program-------------------------
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