From 527c50e07e488d94edcce589528d9f2f5bd928dc Mon Sep 17 00:00:00 2001 From: Chris Allen Date: Sun, 11 Jan 2015 11:39:38 -0600 Subject: [PATCH] new dialogue --- dialogues.md | 48 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/dialogues.md b/dialogues.md index f7a88c6..e0935c8 100644 --- a/dialogues.md +++ b/dialogues.md @@ -1376,3 +1376,51 @@ inv f b = let [a] = [ a | a <- universe, f a == b ] in a ``` The pattern matching above will fail if f is not bijective. + +## pseq, seq, sparks + +``` +16:24 < Lindrian> sparks in haskell seem so... unintuitive +16:24 < Lindrian> It doesnt really follow the traditional sense of how you use threads, which might be why I find it odd +16:25 < qu1j0t3> this might also be a very good thing :-) +16:26 < Lindrian> why? +16:28 < Lindrian> I need to remember the functionality of seq, par and pseq. +16:28 < Lindrian> seq forces evaluation of the first argument, before returning the second? par creates a "spark" that allows the first argument to be evaluated at the same time the second is being returned? +16:28 < Lindrian> I thinkkkkk +16:30 < Lindrian> damn cant remember pseq, gotta look it up +16:31 < Lindrian> oh right, pseq evaluates the left before returning the right, guaranteed. +16:32 < Lindrian> So the difference between seq and pseq is that pseq guarantees it while seq doesnt? +16:51 < qu1j0t3> well, thread API has proven to be a poor fit for many purposes. +16:53 < zwer_z> > seq (error "one") (error "two") +16:53 < lambdabot> *Exception: one +16:53 < Lindrian> > pseq (error "first") (error "second") +16:53 < lambdabot> Not in scope: ‘pseq’ +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 +16:55 < 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 +16:56 < ski> (i think in `seq a b' you're not even guaranteed that `a' is forced before `b' is returned .. as long as it will happen eventually) +16:56 < zwer> ski I think there's a lack of guarantee only if both a and b are bottom. +16:57 < zwer> > seq (error "foo") 10 +16:57 < lambdabot> *Exception: foo +16:57 < zwer> whereas that should always evaluate to bottom +16:58 < benzrf> iirc +16:58 < ski> zwer : consider `seq (seq (error "foo") ()) (error "bar")' -- i think this can legally raise `error "bar"' +16:58 < benzrf> the haskell spec only says +16:58 < benzrf> seq bottom anything = bottom +16:58 < ski> yes +16:58 < benzrf> seq anything bottom = bottom +16:58 < zwer> ski I think so too. if b is bottom a does not have to be evaluated at all +17:00 < ski> zwer : in this case "`b'" for the inner call is not bottom +17:02 < Lindrian> so seq will try to evaluate to WHNF? +17:02 < Lindrian> pseq guarantees WHNF +17:03 < zwer> ski a is entire inner seq, which may not be evaluated +17:05 < ski> yes +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*. `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 < 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 +17:08 < ski> `pseq' would need to be specified using "operational semantics", which talks in terms of rewriting steps (so there's an inherent ordering that we talk about here) +17:08 * ski nods to Lindrian +```