Re: [GHC] #9339: last is not a good consumer

21 Jul 2014

      #9339: last is not a good consumer
-------------------------------------+-------------------------------------
              Reporter:  dfeuer      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:              |           Version:  7.8.3
  libraries/base                     |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:              |   Type of failure:  Runtime
          Architecture:              |  performance bug
  Unknown/Multiple                   |         Test Case:
            Difficulty:  Unknown     |          Blocking:
            Blocked By:              |
       Related Tickets:              |
-------------------------------------+-------------------------------------

Comment (by nomeata):

 Still not a full evaluation, but some more factoids:

 With
 {{{
 myLast2 = foldl (\_ x -> x) undefined
 }}}
 I get
 {{{
 myLast2 :: forall a. [a] -> a
 myLast2 = \ (@ a) -> foldl (myLast1) (undefined)
 }}}
 while when used (in the same model, non-fusing), this gets turned into the
 nice
 {{{
 Rec {
 main_go :: [Int] -> Int -> Int
 main_go =
   \ (ds :: [Int]) (eta :: Int) ->
     case ds of _ {
       [] -> eta;
       : y ys -> main_go ys y
     }
 end Rec }
 }}}

 Writing
 {{{
 myLast2 = inline foldl (\_ x -> x) undefined
 }}}
 also gives
 {{{
 Rec {
 myLast1 :: forall a. [a] -> a -> a
 myLast1 =
   \ (@ a) (ds :: [a]) (eta :: a) ->
     case ds of _ {
       [] -> eta;
       : y ys -> myLast1 ys y
     }
 end Rec }

 myLast2 :: forall a. [a] -> a
 myLast2 = \ (@ a) (xs :: [a]) -> myLast1 xs (undefined)
 }}}

 So it looks good even when not fused. (Measurements are yet pending.)
 for the exported module, but when used, it produce

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9339#comment:5
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler