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@comlab.ox.ac.uk Oxford University Computing Laboratory, TEL: +44 1865 283508 Wolfson Building, Parks Road, FAX: +44 1865 273839 Oxford OX1 3QD, UK. URL: http://www.comlab.ox.ac.uk/oucl/people/jeremy.gibbons.html