[Haskell-cafe] Different semantics in identical do statement?
In my program do x - do blah - someFunc return blah return $ Constructor x behaves differently from do blah - someFunc return $ Constructor blah where the dots are identical. I would think that these programs should behave identically, by the monad laws. The result of my program is that the second gives correct behaviour, while the first loops forever. Greetings, Gerben -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--tp25828319p25828319.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
In what Monad? -Ross On Oct 9, 2009, at 5:43 PM, staafmeister wrote: In my program do x - do blah - someFunc return blah return $ Constructor x behaves differently from do blah - someFunc return $ Constructor blah where the dots are identical. I would think that these programs should behave identically, by the monad laws. The result of my program is that the second gives correct behaviour, while the first loops forever. Greetings, Gerben -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--tp25828319p25828319.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com . ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
Ross Mellgren wrote: In what Monad? Parsec Monad -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--tp25828319p25828521.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
I vaguely remember on IRC someone pointing out that the Parsec monad broke one of the laws. I think return _|_ x === _|_ which could be causing your problem. I may be wrong though. On Fri, Oct 9, 2009 at 5:59 PM, staafmeister g.c.stave...@uu.nl wrote: Ross Mellgren wrote: In what Monad? Parsec Monad -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--tp25828319p25828521.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
Daniel Peebles wrote: I vaguely remember on IRC someone pointing out that the Parsec monad broke one of the laws. I think return _|_ x === _|_ which could be causing your problem. I may be wrong though. This could very well be it. I use lazy eval to construct a function that returns its own argument (that is you supply the return value of the func as its arg). -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--tp25828319p25828612.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
Daniel Peebles wrote:
I vaguely remember on IRC someone pointing out that the Parsec monad
broke one of the laws. I think return _|_ x === _|_ which could be
causing your problem. I may be wrong though.
Confirmed, working in the parsec monad
Prelude Text.Parsec runP (do {x - return undefined; return 10}) ()
*** Exception: Prelude.undefined
In the IO Monad
Prelude Text.Parsec do {x - return undefined; return 10}
10
Should be fixed.
--
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Re: [Haskell-cafe] Different semantics in identical do statement?
On Fri, Oct 9, 2009 at 6:47 PM, staafmeister g.c.stave...@uu.nl wrote:
Daniel Peebles wrote:
I vaguely remember on IRC someone pointing out that the Parsec monad
broke one of the laws. I think return _|_ x === _|_ which could be
causing your problem. I may be wrong though.
Confirmed, working in the parsec monad
Prelude Text.Parsec runP (do {x - return undefined; return 10}) ()
*** Exception: Prelude.undefined
In the IO Monad
Prelude Text.Parsec do {x - return undefined; return 10}
10
Should be fixed.
It looks like the problem is a strict field in the definition of
GenParser (specifically in Reply). From what I can tell, Parsec 3.0.1
should not have this problem.
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] Different semantics in identical do statement?
On Fri, Oct 9, 2009 at 4:25 PM, David Menendez d...@zednenem.com wrote:
On Fri, Oct 9, 2009 at 6:47 PM, staafmeister g.c.stave...@uu.nl wrote:
Daniel Peebles wrote:
I vaguely remember on IRC someone pointing out that the Parsec monad
broke one of the laws. I think return _|_ x === _|_ which could be
causing your problem. I may be wrong though.
Confirmed, working in the parsec monad
Prelude Text.Parsec runP (do {x - return undefined; return 10}) ()
*** Exception: Prelude.undefined
In the IO Monad
Prelude Text.Parsec do {x - return undefined; return 10}
10
Should be fixed.
It looks like the problem is a strict field in the definition of
GenParser (specifically in Reply). From what I can tell, Parsec 3.0.1
should not have this problem.
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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It works with Parsec 3.0.1 for me.
Alex
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