Merge pull request #29 from purescript/alternate

Add Alternate monoid for <|>, empty = <>, mempty

src/Data/Monoid/Alternate.purs

@@ -0,0 +1,58 @@
+module Data.Monoid.Alternate where
+
+import Prelude
+
+import Control.Alternative (class Alt, class Plus, class Alternative, empty, (<|>))
+import Control.Comonad (class Comonad, class Extend)
+
+import Data.Functor.Invariant (class Invariant)
+import Data.Monoid (class Monoid)
+import Data.Newtype (class Newtype)
+
+-- | Monoid and semigroup instances corresponding to `Plus` and `Alt` instances
+-- | for `f`
+-- |
+-- | ``` purescript
+-- | Alternate fx <> Alternate fy == Alternate (fx <|> fy)
+-- | mempty :: Alternate _ == Alternate empty
+-- | ```
+newtype Alternate f a = Alternate (f a)
+
+derive instance newtypeAlternate :: Newtype (Alternate f a) _
+
+derive newtype instance eqAlternate :: Eq (f a) => Eq (Alternate f a)
+
+derive newtype instance ordAlternate :: Ord (f a) => Ord (Alternate f a)
+
+derive newtype instance boundedAlternate :: Bounded (f a) => Bounded (Alternate f a)
+
+derive newtype instance functorAlternate :: Functor f => Functor (Alternate f)
+
+derive newtype instance invariantAlternate :: Invariant f => Invariant (Alternate f)
+
+derive newtype instance applyAlternate :: Apply f => Apply (Alternate f)
+
+derive newtype instance applicativeAlternate :: Applicative f => Applicative (Alternate f)
+
+derive newtype instance altAlternate :: Alt f => Alt (Alternate f)
+
+derive newtype instance plusAlternate :: Plus f => Plus (Alternate f)
+
+derive newtype instance alternativeAlternate :: Alternative f => Alternative (Alternate f)
+
+derive newtype instance bindAlternate :: Bind f => Bind (Alternate f)
+
+derive newtype instance monadAlternate :: Monad f => Monad (Alternate f)
+
+derive newtype instance extendAlternate :: Extend f => Extend (Alternate f)
+
+derive newtype instance comonadAlternate :: Comonad f => Comonad (Alternate f)
+
+instance showAlternate :: Show (f a) => Show (Alternate f a) where
+  show (Alternate a) = "(Alternate " <> show a <> ")"
+
+instance semigroupAlternate :: Alt f => Semigroup (Alternate f a) where
+  append (Alternate a) (Alternate b) = Alternate (a <|> b)
+
+instance monoidAlternate :: Plus f => Monoid (Alternate f a) where
+  mempty = Alternate empty