Data.Rank1Typeable

Runtime type representation of terms with support for rank-1 polymorphic types with type variables of kind *.

The essence of this module is that we use the standard Typeable representation of Data.Typeable but we introduce a special (empty) data type TypVar which represents type variables. TypVar is indexed by an arbitrary other data type, giving you an unbounded number of type variables; for convenience, we define ANY, ANY1, .., ANY9.

Examples of isInstanceOf

-- We CANNOT use a term of type 'Int -> Bool' as 'Int -> Int'
> typeOf (undefined :: Int -> Int) `isInstanceOf` typeOf (undefined :: Int -> Bool)
Left "Cannot unify Int and Bool"

-- We CAN use a term of type 'forall a. a -> Int' as 'Int -> Int'
> typeOf (undefined :: Int -> Int) `isInstanceOf` typeOf (undefined :: ANY -> Int)
Right ()

-- We CAN use a term of type 'forall a b. a -> b' as 'forall a. a -> a'
> typeOf (undefined :: ANY -> ANY) `isInstanceOf` typeOf (undefined :: ANY -> ANY1)
Right ()

-- We CANNOT use a term of type 'forall a. a -> a' as 'forall a b. a -> b'
> typeOf (undefined :: ANY -> ANY1) `isInstanceOf` typeOf (undefined :: ANY -> ANY)
Left "Cannot unify Succ and Zero"

-- We CAN use a term of type 'forall a. a' as 'forall a. a -> a'
> typeOf (undefined :: ANY -> ANY) `isInstanceOf` typeOf (undefined :: ANY)
Right ()

-- We CANNOT use a term of type 'forall a. a -> a' as 'forall a. a'
> typeOf (undefined :: ANY) `isInstanceOf` typeOf (undefined :: ANY -> ANY)
Left "Cannot unify Skolem and ->"

(Admittedly, the quality of the type errors could be improved.)

Examples of funResultTy

-- Apply fn of type (forall a. a -> a) to arg of type Bool gives Bool
> funResultTy (typeOf (undefined :: ANY -> ANY)) (typeOf (undefined :: Bool))
Right Bool

-- Apply fn of type (forall a b. a -> b -> a) to arg of type Bool gives forall a. a -> Bool
> funResultTy (typeOf (undefined :: ANY -> ANY1 -> ANY)) (typeOf (undefined :: Bool))
Right (ANY -> Bool) -- forall a. a -> Bool

-- Apply fn of type (forall a. (Bool -> a) -> a) to argument of type (forall a. a -> a) gives Bool
> funResultTy (typeOf (undefined :: (Bool -> ANY) -> ANY)) (typeOf (undefined :: ANY -> ANY))
Right Bool

-- Apply fn of type (forall a b. a -> b -> a) to arg of type (forall a. a -> a) gives (forall a b. a -> b -> b)
> funResultTy (typeOf (undefined :: ANY -> ANY1 -> ANY)) (typeOf (undefined :: ANY1 -> ANY1))
Right (ANY -> ANY1 -> ANY1)

-- Cannot apply function of type (forall a. (a -> a) -> a -> a) to arg of type (Int -> Bool)
> funResultTy (typeOf (undefined :: (ANY -> ANY) -> (ANY -> ANY))) (typeOf (undefined :: Int -> Bool))
Left "Cannot unify Int and Bool"

Basic types

data TypeRep

Dynamic type representation with support for rank-1 types

Instances

Eq TypeRep

Methods

(==) :: TypeRep -> TypeRep -> Bool

(/=) :: TypeRep -> TypeRep -> Bool

Show TypeRep

Methods

showsPrec :: Int -> TypeRep -> ShowS

show :: TypeRep -> String

showList :: [TypeRep] -> ShowS

Binary TypeRep

Methods

put :: TypeRep -> Put

get :: Get TypeRep

putList :: [TypeRep] -> Put

typeOf :: Typeable a => a -> TypeRep

The type representation of any Typeable term

splitTyConApp :: TypeRep -> (TyCon, [TypeRep])

Split a type representation into the application of a type constructor and its argument

mkTyConApp :: TyCon -> [TypeRep] -> TypeRep

Inverse of splitTyConApp

Operations on type representations

isInstanceOf :: TypeRep -> TypeRep -> Either TypeError ()

t1 isInstanceOf t2 checks if t1 is an instance of t2

funResultTy :: TypeRep -> TypeRep -> Either TypeError TypeRep

funResultTy t1 t2 is the type of the result when applying a function of type t1 to an argument of type t2

type TypeError = String

If isInstanceOf fails it returns a type error

Type variables

data TypVar a

data Zero

data Succ a

type V0 = Zero

type V1 = Succ V0

type V2 = Succ V1

type V3 = Succ V2

type V4 = Succ V3

type V5 = Succ V4

type V6 = Succ V5

type V7 = Succ V6

type V8 = Succ V7

type V9 = Succ V8

type ANY = TypVar V0

type ANY1 = TypVar V1

type ANY2 = TypVar V2

type ANY3 = TypVar V3

type ANY4 = TypVar V4

type ANY5 = TypVar V5

type ANY6 = TypVar V6

type ANY7 = TypVar V7

type ANY8 = TypVar V8

type ANY9 = TypVar V9

Re-exports from Typeable

class Typeable k a

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

Minimal complete definition

typeRep#