On Thu, Sep 08, 2005 at 04:00:47PM +0000, fool@sdf-eu.org wrote:
$ cat bug.hs main = print (let (x,(q,_)) = (1,divMod x x) in q) $ runhugs bug.hs Illegal instruction (core dumped)
Not entirely innocent: demand for x also causes evaluation of the second pair, which depends on x: an infinite loop. The correct version is
main = print (let (x,~(q,_)) = (1,divMod x x) in q)
This is a valuable clue. I previously thought that rewriting let <a> = <b> <c> = <d> in <e> as let (<a>,<c>) = (<b>,<d>) in <e> is harmless, but section 3.12 of the Report says different. I still don't understand why main = print (let (x,(q,_)) = (1,divMod x x) in q) and main = print (let (x,(q,_)) = (1,divmod x x) in q) divmod x y = (div x y,mod x y) behave differently. Please give me another clue. Thank you.