#15566: Implement minimumOn, maximumOn to mirror sortOn -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler