[GHC] #1904: Strictness analyser should be smarter

[GHC]

#1904: Strictness analyser should be smarter
-------------------------------+--------------------------------------------
  Reporter:  igloo             |          Owner:                  
      Type:  bug               |         Status:  closed          
  Priority:  normal            |      Milestone:  _|_             
 Component:  Compiler          |        Version:  6.8.1           
Resolution:  fixed             |       Keywords:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown      |     Difficulty:  Unknown         
  Testcase:                    |      Blockedby:                  
  Blocking:                    |        Related:                  
-------------------------------+--------------------------------------------
Changes (by simonpj):

  * status:  new => closed
  * resolution:  => fixed


Comment:

 The code now looks good.  Here is Case A, with `-ddump-simpl -O` output:
 {{{
 foo :: String -> [String] -> Bool
 foo x _ | x `seq` x == "." = True
 foo x xs                   = x `seq` any (x ==) xs

 =====
 T1904.foo =
   \ (x_aeG :: GHC.Base.String) (ds_dl8 :: [GHC.Base.String]) ->
     case x_aeG of x1_XeK { __DEFAULT ->
     case GHC.Base.eqString x1_XeK T1904.foo1 of _ {
       GHC.Types.False ->
         GHC.List.any @ GHC.Base.String (GHC.Base.eqString x1_XeK) ds_dl8;
       GHC.Types.True -> GHC.Types.True
     } }
 }}}
 Now without the first `seq`, Case B:
 {{{
 foo :: String -> [String] -> Bool
 foo x _ | x == "." = True
 foo x xs           = x `seq` any (x ==) xs

 ====

 T1904.foo =
   \ (x_aeG :: GHC.Base.String) (ds_dl7 :: [GHC.Base.String]) ->
     case GHC.Base.eqString x_aeG T1904.foo1 of _ {
       GHC.Types.False ->
         case x_aeG of x1_akh { __DEFAULT ->
         GHC.List.any @ GHC.Base.String (GHC.Base.eqString x1_akh) ds_dl7
         };
       GHC.Types.True -> GHC.Types.True
     }
 }}}
 Notice that both have one remaining `seq` on `x`, but in one case it's
 before the call to `eqString`, and in the other case after it, which is as
 it should be.

 So I'm happy here, and am closing.  Re-open if you disagree.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1904#comment:6>
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