[GHC] #15566: Implement minimumOn, maximumOn to mirror sortOn

[GHC]

#15566: Implement minimumOn, maximumOn to mirror sortOn
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        Reporter:  Garciat           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  Research
                                     |  needed
       Component:  libraries/base    |              Version:  8.5
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Garciat):

 This is just food for thought:

 {{{#!hs
 import Data.Foldable
 import Data.Semigroup

 newtype Tagged t a = Tagged (t, a)

 instance Eq t => Eq (Tagged t a) where
   Tagged (x, _) == Tagged (y, _) = x == y

 instance Ord t => Ord (Tagged t a) where
   compare (Tagged (x, _)) (Tagged (y, _)) = compare x y

 maximumOn :: (Foldable t, Ord b) => (a -> b) -> t a -> a
 maximumOn f xs =
   case foldMap tag xs of
     Nothing -> error "maximumOn: empty structure"
     Just (Max (Tagged (_, x))) -> x
   where
     tag x = Just . Max . Tagged $ (f x, x)

 minimumOn :: (Foldable t, Ord b) => (a -> b) -> t a -> a
 minimumOn f xs =
   case foldMap tag xs of
     Nothing -> error "minimumOn: empty structure"
     Just (Min (Tagged (_, x))) -> x
   where
     tag x = Just . Min . Tagged $ (f x, x)
 }}}

 I can anticipate the partiality of the functions being a potential problem
 for the general case. I guess that suggests these should be `Foldable`
 methods w/ defaults, instead of standalone functions? And that makes the
 proposal that much hairy due to evolution, etc.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15566#comment:2>
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