Language/Symantic/Type/Maybe.hs

   1 {-# LANGUAGE FlexibleContexts #-}
   2 {-# LANGUAGE GADTs #-}
   3 {-# LANGUAGE MultiParamTypeClasses #-}
   4 {-# LANGUAGE OverloadedStrings #-}
   5 {-# LANGUAGE ScopedTypeVariables #-}
   6 {-# LANGUAGE TypeFamilies #-}
   7 module Language.Symantic.Type.Maybe where
   8
   9 import Data.Maybe (isJust)
  10 import Data.Type.Equality ((:~:)(Refl))
  11
  12 import Language.Symantic.Type.Common
  13 import Language.Symantic.Type.Fun
  14 import Language.Symantic.Type.Bool
  15 import Language.Symantic.Type.Var
  16
  17 -- * Type 'Type_Maybe'
  18 -- | The 'Maybe' type.
  19 data Type_Maybe root h where
  20         Type_Maybe :: root h_a
  21                    -> Type_Maybe root (Maybe h_a)
  22
  23 type instance Root_of_Type      (Type_Maybe root) = root
  24 type instance Error_of_Type ast (Type_Maybe root) = ()
  25
  26 instance -- Type_Eq
  27  Type_Eq root =>
  28  Type_Eq (Type_Maybe root) where
  29         type_eq (Type_Maybe a1) (Type_Maybe a2)
  30          | Just Refl <- a1 `type_eq` a2
  31          = Just Refl
  32         type_eq _ _ = Nothing
  33 instance -- Eq
  34  Type_Eq root =>
  35  Eq (Type_Maybe root h) where
  36         x == y = isJust $ type_eq x y
  37 instance -- String_from_Type
  38  String_from_Type root =>
  39  String_from_Type (Type_Maybe root) where
  40         string_from_type (Type_Maybe a) =
  41                 "Maybe (" ++ string_from_type a ++ ")"
  42 instance -- Show
  43  String_from_Type root =>
  44  Show (Type_Maybe root h) where
  45         show = string_from_type
  46
  47 -- | Convenient alias to include a 'Type_Maybe' within a type.
  48 type_maybe
  49  :: Type_Root_Lift Type_Maybe root
  50  => root h_a
  51  -> root (Maybe h_a)
  52 type_maybe a = type_root_lift (Type_Maybe a)
  53
  54 -- * Type 'Type_Fun_Bool_Maybe'
  55 -- | Convenient alias.
  56 type Type_Fun_Bool_Maybe lam
  57  = Type_Root (Type_Alt Type_Var
  58              (Type_Alt (Type_Fun lam)
  59              (Type_Alt Type_Bool
  60                        Type_Maybe)))