[GHC] #15426: `elemIndex` and `findIndex` still can't fuse

20 Jul 2018

      #15426: `elemIndex` and `findIndex` still can't fuse
-------------------------------------+-------------------------------------
           Reporter:  kabuhr         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:                 |           Version:  8.4.3
  libraries/base                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Ticket #14387 introduced a change to the implementation of `listToMaybe`
 to allow:
 {{{#!hs
 findIndex p = listToMaybe . findIndices p
 }}}
 to fuse.

 However, to take advantage of this, it looks like we also need `findIndex`
 (and `elemIndex`) to be marked inlinable (or some similar step).

 As a concrete example, the module:

 {{{#!hs
 module Foo where

 import Data.List (findIndex)

 foo :: Maybe Int
 foo = findIndex (==999999) [1..1000000]
 }}}

 compiled with GHC 8.4.3 using `-O2` produces the following unfused core:

 {{{#!hs
 foo_go
   = \ ds1_a2ws eta_a2wt ->
       case ds1_a2ws of {
         [] -> Nothing;
         : y_a2wx ys_a2wy ->
           case eqInteger# y_a2wx ds_r2we of {
             __DEFAULT -> foo_go ys_a2wy (+# eta_a2wt 1#);
             1# -> Just (I# eta_a2wt)
           }
       }

 foo = foo_go (enumDeltaToInteger1 foo2 foo1) 0#
 }}}
 but if the definition of `findIndex` from `Data.OldList` is copied into
 the module or imported from another module with an `INLINABLE` pragma,
 then it fuses fine:
 {{{#!hs
 foo_go
   = \ x_a2Du eta_B1 ->
       case gtInteger# x_a2Du lim_r2Ey of {
         __DEFAULT ->
           case eqInteger# x_a2Du ds_r2Cv of {
             __DEFAULT -> foo_go (plusInteger x_a2Du foo1) (+# eta_B1 1#);
             1# -> Just (I# eta_B1)
           };
         1# -> Nothing
       }

 foo = foo_go foo1 0#
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15426
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