strictness question

[Simon Peyton-Jones]

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
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