[Haskell-cafe] Is there a type class for boring types?
David Feuer
david.feuer at gmail.com
Tue Feb 2 17:41:34 UTC 2016
Or, alternatively, some common class that lets me express that a type is
boring (i.e., inhabited by precisely one fully-defined value)? lens has
Settable, whose law ensures the type involved has a boring representation
(in the sense of representable functor), but is there a more fundamental
way?
class Boring x where
inhabitant :: x
instance Boring () where
inhabitant = ()
instance Boring (Proxy a) where
inhabitant = Proxy
instance Boring y => Boring (x -> y) where
inhabitant = const inhabitant
instance (Boring x, Boring y) => Boring (x, y) where
inhabitant = (inhabitant, inhabitant)
instance Boring x => Boring (Const x y) where
inhabitant = Const inhabitant
instance Boring x => Boring (Identity x) where
inhabitant = Identity inhabitant
...
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