factoring `if'

[Carsten Schultz]

Hi!

On Mon, Oct 11, 2004 at 02:19:45PM +0400, Serge D. Mechveliani wrote:
> Dear Haskell implementors,
> 
> How do you think, is the program (1) equivalent to (2) 
> in the meaning of Haskell-98 ?
> 
> (1)   (\ x -> (if p x then  foo (g x)  else  foo (h x))
>               where
>               p ... g ... h ... foo ...
>       )
> 
> (2)   (\ x -> foo  ((if p x then  g x  else  h x)
>                     where
>                     p ... g ... h ... foo ...
>                    )
>       )

In general they are not, for exactly the reason you give:

> The reason for this may be, for example, that the result printing
> of  (f x)  is more `lazy' in (2) than in (1):
> the part of  foo  may print immediately and  (g x) or (h x)  may print
> long after.
> This is a difference in behavior, it does not effect the computation
> meaning.

If  p x == _|_  and  foo _|_ /= _|_, then (1) does not equal (2).

> I have a large program which is easily written in the style of (1),
> (and in many places it sets `case' instead of `if').  
> Annoyingly, it prints out in a not a lazy manner.

You get what you specify :-)

Greetings,

Carsten

-- 
Carsten Schultz (2:38, 33:47), FB Mathematik, FU Berlin
http://carsten.codimi.de/
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