Language/Symantic/Type/Maybe.hs

   1 {-# LANGUAGE FlexibleContexts #-}
   2 {-# LANGUAGE FlexibleInstances #-}
   3 {-# LANGUAGE PatternSynonyms #-}
   4 {-# LANGUAGE TypeFamilies #-}
   5 {-# OPTIONS_GHC -fno-warn-orphans #-}
   6 module Language.Symantic.Type.Maybe where
   7
   8 import Data.Proxy
   9 import Data.Type.Equality ((:~:)(Refl))
  10 import Language.Symantic.Type.Common
  11
  12 -- * Type 'Type_Maybe'
  13 -- | The 'Maybe' type.
  14 type Type_Maybe = Type_Type1 (Proxy Maybe)
  15
  16 pattern Type_Map :: root a -> Type_Maybe root (Maybe a)
  17 pattern Type_Map a = Type_Type1 Proxy a
  18
  19 instance -- Eq_Type
  20  Eq_Type root =>
  21  Eq_Type (Type_Type1 (Proxy Maybe) root) where
  22         eq_type (Type_Type1 _px1 a1) (Type_Type1 _px2 a2)
  23          | Just Refl <- a1 `eq_type` a2
  24          = Just Refl
  25         eq_type _ _ = Nothing
  26 instance -- Eq_Type1
  27  Eq_Type1 (Type_Type1 (Proxy Maybe) root) where
  28         eq_type1 Type_Type1{} Type_Type1{}
  29          = Just Refl
  30 instance -- String_from_Type
  31  String_from_Type root =>
  32  String_from_Type (Type_Maybe root) where
  33         string_from_type (Type_Type1 _f a) =
  34                 "Maybe" ++ " (" ++ string_from_type a ++ ")"
  35
  36 -- | Inject 'Type_Maybe' within a root type.
  37 type_maybe
  38  :: Lift_Type_Root Type_Maybe root
  39  => root h_a
  40  -> root (Maybe h_a)
  41 type_maybe = lift_type_root . Type_Type1 (Proxy::Proxy Maybe)