[GHC] #15182: Lazier Semigroup instance for Maybe

[GHC]

#15182: Lazier Semigroup instance for Maybe
-------------------------------------+-------------------------------------
           Reporter:  andrewthad     |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           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:
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 Mailing list discussion here:
 https://mail.haskell.org/pipermail/libraries/2018-May/028818.html

 The existing `Semigroup` instance for `Maybe` is:

 {{{
 instance Semigroup a => Semigroup (Maybe a) where
     Nothing <> b       = b
     a       <> Nothing = a
     Just a  <> Just b  = Just (a <> b)
 }}}

 It has been proposed that it be replaced by:

 {{{
 instance Semigroup a => Semigroup (Maybe a) where
     Nothing <> b = b
     Just a  <> b = Just (maybe a (a<>) b)
 }}}

 This is lazier in the second argument, making it more consistent with the
 strictness of the `Semigroup` instances for
 `And`,`Or`,`Ord`,`Either`,`Proxy` and `()`. Equivalently, we could write:

 {{{
 instance Semigroup a => Semigroup (Maybe a) where
     Nothing <> b = b
     Just a  <> Nothing = Just a
     Just a  <> Just b  = Just (a <> b)
 }}}

 This makes it a little more clear that we aren't building a closure for
 partial function application, but it should have the same behavior.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15182>
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