#9759: Add Alternative wrapper to Data.Monoid -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: Core Libraries | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 | Type of failure: hour) | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- As discussed on the libraries list, {{{#!hs -- | Monoid under @<|>@. newtype Alt f a = Alt {getAlt :: f a} deriving (Generic, Generic1, Read, Show, Eq, Ord, Num, Enum, Monad, MonadPlus, Applicative, Alternative, Functor) instance forall f a . Alternative f => Monoid (Alt f a) where mempty = Alt empty mappend = coerce ((<|>) :: f a -> f a -> f a) }}} The documentation for `Data.Monoid.First` and `Data.Monoid.Last` should also be expanded to explain that `First a` is isomorphic to `Alt Maybe a` and `Last a` is isomorphic to `Dual (Alt Maybe a)`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9759 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler