Strange. You don't supply a complete program, so it's hard to test. Nevertheless, the Haskell Report (Sect 3.12) specifies that a let adds a single twiddle. Thus let (x, (y,z)) = e in b means let x = case e of (x,(y,z)) -> x y = case e of (x,(y,z)) -> y z = case e of (x,(y,z)) -> z in b And that is what GHC implements. You get something different if you add twiddles inside: let (x, ~(y,z)) = e in b means let x = case e of (x,_) -> x y = case e of (_,(y,_)) -> y etc Adding more twiddles means less eager matching. I don't know whether Hugs implements this. Simon | -----Original Message----- | From: S. Doaitse Swierstra [mailto:doaitse@cs.uu.nl] | Sent: 01 March 2001 11:26 | To: haskell@haskell.org | Subject: strictness question | | | 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 | | | | | -- | ______________________________________________________________ | ____________ | S. Doaitse Swierstra, Department of Computer Science, Utrecht | University | P.O.Box 80.089, 3508 TB UTRECHT, the | Netherlands | Mail: mailto:doaitse@cs.uu.nl | WWW: http://www.cs.uu.nl/ | PGP Public Key: http://www.cs.uu.nl/people/doaitse/ tel: +31 (30) 253 3962, fax: +31 (30) 2513791 __________________________________________________________________________ _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell