[GHC] #15131: Speed up certain Foldable NonEmpty methods

[GHC]

#15131: Speed up certain Foldable NonEmpty methods
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Core           |           Version:  8.2.2
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Yitzchak Gale pointed out
 [https://www.reddit.com/r/haskell/comments/8hxpns/why_is_nonempty_missing_some_foldable_method/
 on Reddit] that some `Foldable` methods are allowed to take default
 definitions for `NonEmpty` when they probably shouldn't. The biggest
 problem appears to be `foldr1`, which ends up with a fairly atrocious
 definition. Yitz seems to think we should also provide a custom definition
 of `null`, to avoid relying on optimizations, but at present there is no
 obvious need for that. He doesn't mention this, but the definition of
 `length` should be changed. Indeed, the default definition of `length`
 would be better than the custom definition we currently have. I also
 noticed that we don't mark `foldl1` `INLINE`. At present its unfolding is
 optimized, so its `foldr` is not exposed. Thus, for example,

 {{{#!hs
 foldl1 (+) $ 1 :| [2..n]
 }}}

 won't fuse. We can certainly mark `foldl1` `INLINE` to fix this
 (`INLINABLE` doesn't do the trick), but it would be nice to know if
 there's another way.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15131>
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