WIP: beginning of a module with a concrete representation of Type and conversions

TemplateHaskell/Syntax/Datatypes/Type.hs

+-- | This module defines a concrete representation of the syntax of types, and
+-- provides conversion functions between the concrete representation and the
+-- compiler-internal representation (which is intended to remain abstract).
+--
+module TemplateHaskell.Syntax.Datatypes.Type
+  ( Type(..)
+  , Specificity(..)
+  , TyVarBndr(..)
+  , Kind
+  , SumArity
+  , Cxt
+  , Pred
+
+  , reifyType
+  , reflectType
+  )
+  where
+
+import Data.Data (Data)
+import GHC.Generics (Generic)
+import GHC.Internal.TH.Syntax (Name, Q)
+import qualified GHC.Internal.TH.Syntax as Internal
+import qualified GHC.Internal.TH.Ppr as Ppr
+
+-- | A Haskell type.
+data Type = ForallT [TyVarBndr Specificity] Cxt Type -- ^ @forall \<vars\>. \<ctxt\> => \<type\>@
+          | ForallVisT [TyVarBndr ()] Type -- ^ @forall \<vars\> -> \<type\>@
+          | AppT Type Type                 -- ^ @T a b@
+          | AppKindT Type Kind             -- ^ @T \@k t@
+          | SigT Type Kind                 -- ^ @t :: k@
+          | VarT Name                      -- ^ @a@
+          | ConT Name                      -- ^ @T@
+          | PromotedT Name                 -- ^ @'T@
+          | InfixT Type Name Type          -- ^ @T + T@
+          | UInfixT Type Name Type         -- ^ @T + T@
+                                           --
+                                           -- See "Language.Haskell.TH.Syntax#infix"
+          | PromotedInfixT Type Name Type  -- ^ @T :+: T@
+          | PromotedUInfixT Type Name Type -- ^ @T :+: T@
+                                           --
+                                           -- See "Language.Haskell.TH.Syntax#infix"
+          | ParensT Type                   -- ^ @(T)@
+
+          -- See Note [Representing concrete syntax in types]
+          | TupleT Int                     -- ^ @(,)@, @(,,)@, etc.
+          | UnboxedTupleT Int              -- ^ @(\#,\#)@, @(\#,,\#)@, etc.
+          | UnboxedSumT SumArity           -- ^ @(\#|\#)@, @(\#||\#)@, etc.
+          | ArrowT                         -- ^ @->@
+          | MulArrowT                      -- ^ @%n ->@
+                                           --
+                                           -- Generalised arrow type with multiplicity argument
+          | EqualityT                      -- ^ @~@
+          | ListT                          -- ^ @[]@
+          | PromotedTupleT Int             -- ^ @'()@, @'(,)@, @'(,,)@, etc.
+          | PromotedNilT                   -- ^ @'[]@
+          | PromotedConsT                  -- ^ @'(:)@
+          | StarT                          -- ^ @*@
+          | ConstraintT                    -- ^ @Constraint@
+          | LitT TyLit                     -- ^ @0@, @1@, @2@, etc.
+          | WildCardT                      -- ^ @_@
+          | ImplicitParamT String Type     -- ^ @?x :: t@
+      deriving( Show, Eq, Ord, Data, Generic )
+
+-- | The specificity of a type variable in a @forall ...@.
+data Specificity = SpecifiedSpec          -- ^ @a@
+                 | InferredSpec           -- ^ @{a}@
+      deriving( Show, Eq, Ord, Data, Generic )
+
+-- | The @flag@ type parameter is instantiated to one of the following types:
+--
+--   * 'Specificity' (examples: 'ForallC', 'ForallT')
+--   * 'BndrVis' (examples: 'DataD', 'ClassD', etc.)
+--   * '()', a catch-all type for other forms of binders, including 'ForallVisT', 'DataInstD', 'RuleP', and 'TyVarSig'
+--
+data TyVarBndr flag = PlainTV  Name flag      -- ^ @a@
+                    | KindedTV Name flag Kind -- ^ @(a :: k)@
+      deriving( Show, Eq, Ord, Data, Generic, Functor, Foldable, Traversable )
+
+
+-- | To avoid duplication between kinds and types, they
+-- are defined to be the same. Naturally, you would never
+-- have a type be 'StarT' and you would never have a kind
+-- be 'SigT', but many of the other constructors are shared.
+-- Note that the kind @Bool@ is denoted with 'ConT', not
+-- 'PromotedT'. Similarly, tuple kinds are made with 'TupleT',
+-- not 'PromotedTupleT'.
+
+type Kind = Type
+
+-- | In 'UnboxedSumE', 'UnboxedSumT', and 'UnboxedSumP', the total number of
+-- 'SumAlt's. For example, @(\#|\#)@ has a 'SumArity' of 2.
+type SumArity = Int
+
+-- | A context, as found on the left side of a @=>@ in a type.
+type Cxt = [Pred]                 -- ^ @(Eq a, Ord b)@
+
+-- | Since the advent of @ConstraintKinds@, constraints are really just types.
+-- Equality constraints use the 'EqualityT' constructor. Constraints may also
+-- be tuples of other constraints.
+type Pred = Type
+
+-- | Type-level literals.
+data TyLit = NumTyLit Integer             -- ^ @2@
+           | StrTyLit String              -- ^ @\"Hello\"@
+           | CharTyLit Char               -- ^ @\'C\'@, @since 4.16.0.0
+  deriving ( Show, Eq, Ord, Data, Generic )
+
+
+-- | Convert the internal abstract representation of types used by the compiler
+-- into a concrete 'Type'.
+--
+-- This may fail if the type being reified uses a newly-introduced feature that
+-- is not representable in the concrete representation supported by this version
+-- of the library.
+--
+reifyType :: MonadFail m => Internal.Type -> m Type
+reifyType (Internal.VarT n) = pure $ VarT n
+reifyType t = fail $ "reifyType does not support: " ++ Ppr.pprint t
+
+-- | Convert a concrete 'Type' to the internal abstract representation of types
+-- used by the compiler.
+--
+-- This may fail if the type being reflected uses a deprecated feature that is
+-- no longer representable in the abstract representation.
+--
+reflectType :: MonadFail m => Type -> m Internal.Type
+reflectType (VarT n) = pure $ Internal.VarT n

template-haskell-next.cabal

@@ -42,6 +42,7 @@ Library
         TemplateHaskell.Splice
         TemplateHaskell.Syntax
         TemplateHaskell.Syntax.Datatypes
+        TemplateHaskell.Syntax.Datatypes.Type
         TemplateHaskell.Syntax.Monadic
         TemplateHaskell.Typed
         TemplateHaskell.Typed.CodeDo