Language/Symantic/Type/Unit.hs

   1 {-# LANGUAGE FlexibleContexts #-}
   2 {-# LANGUAGE GADTs #-}
   3 {-# LANGUAGE MultiParamTypeClasses #-}
   4 {-# LANGUAGE OverloadedStrings #-}
   5 {-# LANGUAGE ScopedTypeVariables #-}
   6 {-# LANGUAGE TypeFamilies #-}
   7 {-# LANGUAGE UndecidableInstances #-}
   8 module Language.Symantic.Type.Unit where
   9
  10 import Data.Maybe (isJust)
  11 import Data.Type.Equality ((:~:)(Refl))
  12
  13 import Language.Symantic.Type.Common
  14
  15 -- * Type 'Type_Unit'
  16 -- | The unit type.
  17 data Type_Unit (root:: * -> *) h where
  18         Type_Unit :: Type_Unit root ()
  19 type instance Root_of_Type      (Type_Unit root) = root
  20 type instance Error_of_Type ast (Type_Unit root) = ()
  21
  22 instance -- Type_Eq
  23  Type_Eq (Type_Unit root) where
  24         type_eq Type_Unit Type_Unit = Just Refl
  25 instance -- Eq
  26  Eq (Type_Unit root h) where
  27         x == y = isJust $ x `type_eq` y
  28 instance -- String_from_Type
  29  String_from_Type (Type_Unit root) where
  30         string_from_type Type_Unit = "Unit"
  31 instance -- Show
  32  Show (Type_Unit root h) where
  33         show = string_from_type
  34 instance Type_Constraint Eq (Type_Unit root) where
  35         type_constraint _c Type_Unit = Just Dict
  36 instance Type_Constraint Ord (Type_Unit root) where
  37         type_constraint _c Type_Unit = Just Dict
  38
  39 -- | Convenient alias to include a 'Type_Unit' within a type.
  40 type_unit :: Type_Root_Lift Type_Unit root => root ()
  41 type_unit = type_root_lift Type_Unit