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Chris Allen 2015-01-11 11:43:01 -06:00
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@ -1399,7 +1399,8 @@ second is being returned?
16:53 < Lindrian> > pseq (error "first") (error "second") 16:53 < Lindrian> > pseq (error "first") (error "second")
16:53 < lambdabot> Not in scope: ‘pseq’ 16:53 < lambdabot> Not in scope: ‘pseq’
16:53 < lambdabot> Perhaps you meant ‘seq’ (imported from Prelude) 16:53 < lambdabot> Perhaps you meant ‘seq’ (imported from Prelude)
16:54 < zwer_z> IIRC in ghc seq will always evaluate to error "one", while haskell (the language) allows either 16:54 < zwer_z> IIRC in ghc seq will always evaluate to error "one", while
haskell (the language) allows either
16:55 < Lindrian> i read seq as "try to evaluate the first argument, but compiler might think otherwise" 16:55 < Lindrian> i read seq as "try to evaluate the first argument, but compiler might think otherwise"
16:55 < ski> Lindrian : yes, `pseq a b' guarantees `a' is forced before `b'. `seq a b' doesn't 16:55 < ski> Lindrian : yes, `pseq a b' guarantees `a' is forced before `b'. `seq a b' doesn't
16:56 < ski> (i think in `seq a b' you're not even guaranteed that `a' is forced before `b' is 16:56 < ski> (i think in `seq a b' you're not even guaranteed that `a' is forced before `b' is
@ -1424,10 +1425,11 @@ raise `error "bar"'
17:05 < ski> (i think i used a different argument, though. i can't recall it atm) 17:05 < ski> (i think i used a different argument, though. i can't recall it atm)
17:06 < ski> Lindrian : both will evaluate to WHNF. `pseq' guarantees sequential *ordering*. 17:06 < ski> Lindrian : both will evaluate to WHNF. `pseq' guarantees sequential *ordering*.
`seq' just guarantees that both will be forced `seq' just guarantees that both will be forced
17:07 < Lindrian> pseq guarantees the first argument is evaluated to WHNF before the second, while seq doesnt. Ok. 17:07 < Lindrian> pseq guarantees the first argument is evaluated to WHNF before the second,
while seq doesnt. Ok.
17:07 < ski> `seq' is specified using "denotational semantics", which only talks about the 17:07 < ski> `seq' is specified using "denotational semantics", which only talks about the
final value/denotation of an evaluation, not how you can get to it final value/denotation of an evaluation, not how you can get to it
17:08 < ski> `pseq' would need to be specified using "operational semantics", which talks in terms of 17:08 < ski> `pseq' would need to be specified using "operational semantics", which talks
rewriting steps (so there's an inherent ordering that we talk about here) in terms of rewriting steps (so there's an inherent ordering that we talk about here)
17:08 * ski nods to Lindrian 17:08 * ski nods to Lindrian
``` ```