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49 lines
1.5 KiB
Idris
49 lines
1.5 KiB
Idris
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module Data.Contravariant
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infixl 4 >$, >$<, >&<, $<
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||| Contravariant functors
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public export
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interface Contravariant (0 f : Type -> Type) where
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contramap : (a -> b) -> f b -> f a
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(>$) : b -> f b -> f a
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v >$ fb = contramap (const v) fb
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||| If `f` is both `Functor` and `Contravariant` then by the time you factor in the
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||| laws of each of those classes, it can't actually use its argument in any
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public export %inline
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phantom : (Functor f, Contravariant f) => f a -> f b
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phantom fa = () >$ (fa $> ())
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||| This is an infix alias for `contramap`.
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public export %inline
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(>$<) : Contravariant f => (a -> b) -> f b -> f a
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(>$<) = contramap
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||| This is an infix version of `contramap` with the arguments flipped.
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public export %inline
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(>&<) : Contravariant f => f b -> (a -> b) -> f a
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(>&<) = flip contramap
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||| This is `>$` with its arguments flipped.
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public export %inline
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($<) : Contravariant f => f b -> b -> f a
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($<) = flip (>$)
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||| Composition of `Contravariant` is a `Functor`.
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public export
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[Compose] (Contravariant f, Contravariant g) => Functor (f . g) where
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map = contramap . contramap
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||| Composition of a `Functor` and a `Contravariant` is a `Contravariant`.
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public export
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[ComposeFC] (Functor f, Contravariant g) => Contravariant (f . g) where
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contramap = map . contramap
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||| Composition of a `Contravariant` and a `Functor` is a `Contravariant`.
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public export
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[ComposeCF] (Contravariant f, Functor g) => Contravariant (f . g) where
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contramap = contramap . map
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