Data/Type/Bool.hs

    {-# LANGUAGE TypeFamilyDependencies, Safe, PolyKinds #-}
    {-# LANGUAGE TypeFamilies, TypeOperators, DataKinds, NoImplicitPrelude #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Type.Bool
    -- License     :  BSD-style (see the LICENSE file in the distribution)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  not portable
    --
    -- Basic operations on type-level Booleans.
    --
    -- @since 4.7.0.0
    -----------------------------------------------------------------------------

    module Data.Type.Bool (
      If, type (&&), type (||), Not
      ) where

    import Data.Bool

    -- This needs to be in base because (&&) is used in Data.Type.Equality.
    -- The other functions do not need to be in base, but seemed to be appropriate
    -- here.

    -- | Type-level "If". @If True a b@ ==> @a@; @If False a b@ ==> @b@
    type family If cond tru fls where
      If 'True  tru  fls = tru
      If 'False tru  fls = fls

    -- | Type-level "and"
    type family a && b where
      'False && a      = 'False
      'True  && a      = a
      a      && 'False = 'False
      a      && 'True  = a
      a      && a      = a
    infixr 3 &&

    -- | Type-level "or"
    type family a || b where
      'False || a      = a
      'True  || a      = 'True
      a      || 'False = a
      a      || 'True  = 'True
      a      || a      = a
    infixr 2 ||

    -- | Type-level "not". An injective type family since @4.10.0.0@.
    --
    -- @since 4.7.0.0
    type family Not a = res | res -> a where
      Not 'False = 'True
      Not 'True  = 'False