strictness question

[S. Doaitse Swierstra]

I ran into a difference between GHC and Hugs. The following code:

f  (P p) ~(P q)   = P (\ k -> \inp -> let (((pv, (qv, r)), m), st) = 
p (q k) inp
                                       in  (((pv qv  , r ), m), st))

runs fine with Hugs but blows up with GHC, whereas:

f  (P p) ~(P q)   = P (\ k -> \inp -> let ~(~(~(pv, ~(qv, r)), m), 
st) = p (q k) inp
                                       in  (((pv qv  , r ), m), st))

runs fine with GHC too.

 From the Haskell manual I understand that pattern matching in "let"'s 
should be done lazily, so the addition of a collection of ~'s should 
not make a difference. Am I right with  this interpretation?

A possible source of this problem may be origination from the smarter 
GHC optimiser, but in that case the optimiser is not doing its work 
well.

    Doaitse Swierstra




-- 
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S. Doaitse Swierstra, Department of Computer Science, Utrecht University
                       P.O.Box 80.089, 3508 TB UTRECHT,   the Netherlands
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