Question about BangPatterns semantics/documentation

[Simon Peyton Jones]

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 at haskell.org] On Behalf Of
| Joachim Breitner
| Sent: 03 August 2014 15:41
| To: ghc-devs at 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 at joachim-breitner.de • http://www.joachim-breitner.de/
|   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
|   Debian Developer: nomeata at debian.org