Yes, Joachim is dead right. In Haskell (case f x of y -> blah) really is equivalent to (let y = f x in blah). Herbert, if you think a reminder of this point, in the documentation or user manual, would be helpful, please suggest what and where. Simon | -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of | Joachim Breitner | Sent: 03 August 2014 15:41 | To: ghc-devs@haskell.org | Subject: Re: Question about BangPatterns semantics/documentation | | Hi Herbert, | | Am Sonntag, den 03.08.2014, 11:31 +0200 schrieb Herbert Valerio Riedel: | > However, the following two functions are not equivalent after | > compilation to Core: | > | > g, h :: (Int -> Int) -> Int -> () | > g f x = let !y = f x in () | > h f x = case f x of y -> () | > | > In fact, compilation results in | > | > g = \ (f_asi :: Int -> Int) | > (x_asj :: Int) -> | > case f_asi x_asj of _ [Occ=Dead] { I# ipv_sKS -> () } | > | > h = \ _ [Occ=Dead] _ [Occ=Dead] -> () | > | > Is the documentation inaccurate/incomplete/I-missed-something or is | > the implementation to blame? | | I think that in Haskell (which is not Core!), a "case" does not imply | evaluation – only if the patterns require it. So the example in the | docs is correct (case e of [x,y] -> b requires evaluation of e), but | your example is simply optimized away. | | haskell.org is down, so I can’t check if the report has anything to say | about that. | | Greetings, | Joachim | | -- | Joachim “nomeata” Breitner | mail@joachim-breitner.de • http://www.joachim-breitner.de/ | Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F | Debian Developer: nomeata@debian.org