Type.Reflection.Compat

A type application.

For instance,

typeRep @(Maybe Int) === App (typeRep @Maybe) (typeRep @Int)

Note that this will also match a function type,

typeRep @(Int# -> Char)
  ===
App (App arrow (typeRep @Int#)) (typeRep @Char)

where arrow :: TypeRep ((->) :: TYPE IntRep -> Type -> Type).

Pattern match on a type constructor

Pattern match on a type constructor including its instantiated kind variables.

For instance,

App (Con' proxyTyCon ks) intRep = typeRep @(Proxy @Int)

will bring into scope,

proxyTyCon :: TyCon
ks         == [someTypeRep Type] :: [SomeTypeRep]
intRep     == typeRep Int

The function type constructor.

For instance,

typeRep @(Int -> Char) === Fun (typeRep @Int) (typeRep @Char)

A explicitly bidirectional pattern synonym to construct a concrete representation of a type.

As an expression: Constructs a singleton TypeRep a given a implicit 'Typeable a' constraint:

TypeRep @a :: Typeable a => TypeRep a

As a pattern: Matches on an explicit TypeRep a witness bringing an implicit Typeable a constraint into scope.

f :: TypeRep a -> ..
f TypeRep = {- Typeable a in scope -}

Type equality decision

Type equality

Helper to fully evaluate TyCon for use as NFData(rnf) implementation

Helper to fully evaluate SomeTypeRep for use as NFData(rnf) implementation

Helper to fully evaluate TypeRep for use as NFData(rnf) implementation

Takes a value of type a and returns a concrete representation of that type.

Observe the type constructor of a quantified type representation.

Observe the kind of a type.

Observe the type constructor of a type representation

Use a TypeRep as Typeable evidence.

The TypeRep pattern synonym brings a Typeable constraint into scope and can be used in place of withTypeable.

f :: TypeRep a -> ..
f rep = withTypeable {- Typeable a in scope -}

f :: TypeRep a -> ..
f TypeRep = {- Typeable a in scope -}

Propositional equality. If a :~: b is inhabited by some terminating value, then the type a is the same as the type b. To use this equality in practice, pattern-match on the a :~: b to get out the Refl constructor; in the body of the pattern-match, the compiler knows that a ~ b.

Kind heterogeneous propositional equality. Like :~:, a :~~: b is inhabited by a terminating value if and only if a is the same type as b.

A non-indexed type representation.

TypeRep is a concrete representation of a (monomorphic) type. TypeRep supports reasonably efficient equality. See Note [Grand plan for Typeable] in GHC.Tc.Instance.Typeable

The class Typeable allows a concrete representation of a type to be calculated.