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Add Semigroup (Predicate a) instance #39

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Jan 8, 2018
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Add Semigroup (Predicate a) instance #39

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2 changes: 2 additions & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
@@ -1,2 +1,4 @@
_darcs
dist
dist-newstyle
.ghc.environment.*
9 changes: 9 additions & 0 deletions src/Data/Functor/Contravariant.hs
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Expand Up @@ -310,9 +310,18 @@ newtype Predicate a = Predicate { getPredicate :: a -> Bool }
instance Contravariant Predicate where
contramap f g = Predicate $ getPredicate g . f

#if defined(MIN_VERSION_semigroups) || __GLASGOW_HASKELL__ >= 711
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Use MIN_VERSION_base(4,9,0) instead of __GLASGOW_HASKELL__ >= 711.

instance Semigroup (Predicate a) where
Predicate p <> Predicate q = Predicate $ \a -> p a && q a
#endif

instance Monoid (Predicate a) where
mempty = Predicate $ const True
#if defined(MIN_VERSION_semigroups) || __GLASGOW_HASKELL__ >= 711
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Same here.

mappend = (<>)
#else
mappend (Predicate p) (Predicate q) = Predicate $ \a -> p a && q a
#endif

-- | Defines a total ordering on a type as per 'compare'
--
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