[Haskell] factoring `if'

[Jeremy Gibbons]

On Mon, 11 Oct 2004, Serge D. Mechveliani wrote:

> How do you think, is the program (1) equivalent to (2)
> in the meaning of Haskell-98 ?

Not at all. If foo is non-strict and p partial, (2) may yield a result
where (1) would not. You identify the possibility yourself: (2) is lazier.

> (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 ...
>                    )
>       )
>
> If it is equivalent, then does it make sense for a compiler to
> convert (1) to (2):  to separate a common `factor' of the if-branches
> ?
> 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.
>
> 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.
> It can be rewritten in the form of (2), but with effort, and it will
> look less plain.
> So, maybe, this is a business of a compiler?

Jeremy.Gibbons at comlab.ox.ac.uk
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