
Data.Typeable  Portability  portable  Stability  experimental  Maintainer  libraries@haskell.org 





Description 
The Typeable class reifies types to some extent by associating type
representations to types. These type representations can be compared,
and one can in turn define a typesafe cast operation. To this end,
an unsafe cast is guarded by a test for type (representation)
equivalence. The module Data.Dynamic uses Typeable for an
implementation of dynamics. The module Data.Generics uses Typeable
and typesafe cast (but not dynamics) to support the "Scrap your
boilerplate" style of generic programming.


Synopsis 

class Typeable a where    cast :: (Typeable a, Typeable b) => a > Maybe b   gcast :: (Typeable a, Typeable b) => c a > Maybe (c b)   data TypeRep   data TyCon   showsTypeRep :: TypeRep > ShowS   mkTyCon :: String > TyCon   mkTyConApp :: TyCon > [TypeRep] > TypeRep   mkAppTy :: TypeRep > TypeRep > TypeRep   mkFunTy :: TypeRep > TypeRep > TypeRep   splitTyConApp :: TypeRep > (TyCon, [TypeRep])   funResultTy :: TypeRep > TypeRep > Maybe TypeRep   typeRepTyCon :: TypeRep > TyCon   typeRepArgs :: TypeRep > [TypeRep]   tyConString :: TyCon > String   typeRepKey :: TypeRep > IO Int   class Typeable1 t where    class Typeable2 t where    class Typeable3 t where    class Typeable4 t where    class Typeable5 t where    class Typeable6 t where    class Typeable7 t where    gcast1 :: (Typeable1 t, Typeable1 t') => c (t a) > Maybe (c (t' a))   gcast2 :: (Typeable2 t, Typeable2 t') => c (t a b) > Maybe (c (t' a b))   typeOfDefault :: (Typeable1 t, Typeable a) => t a > TypeRep   typeOf1Default :: (Typeable2 t, Typeable a) => t a b > TypeRep   typeOf2Default :: (Typeable3 t, Typeable a) => t a b c > TypeRep   typeOf3Default :: (Typeable4 t, Typeable a) => t a b c d > TypeRep   typeOf4Default :: (Typeable5 t, Typeable a) => t a b c d e > TypeRep   typeOf5Default :: (Typeable6 t, Typeable a) => t a b c d e f > TypeRep   typeOf6Default :: (Typeable7 t, Typeable a) => t a b c d e f g > TypeRep 



The Typeable class



The class Typeable allows a concrete representation of a type to
be calculated.
  Methods   Takes a value of type a and returns a concrete representation
of that type. The value of the argument should be ignored by
any instance of Typeable, so that it is safe to pass undefined as
the argument.

  Instances  


Typesafe cast



The typesafe cast operation



A flexible variation parameterised in a type constructor


Type representations



A concrete representation of a (monomorphic) type. TypeRep
supports reasonably efficient equality.
 Instances  



An abstract representation of a type constructor. TyCon objects can
be built using mkTyCon.
 Instances  




Construction of type representations



:: String  the name of the type constructor (should be unique
in the program, so it might be wise to use the
fully qualified name).
 > TyCon  A unique TyCon object
 Builds a TyCon object representing a type constructor. An
implementation of Data.Typeable should ensure that the following holds:
mkTyCon "a" == mkTyCon "a"




Applies a type constructor to a sequence of types



Adds a TypeRep argument to a TypeRep.



A special case of mkTyConApp, which applies the function
type constructor to a pair of types.


Observation of type representations



Splits a type constructor application



Applies a type to a function type. Returns: Just u if the
first argument represents a function of type t > u and the
second argument represents a function of type t. Otherwise,
returns Nothing.



Observe the type constructor of a type representation



Observe the argument types of a type representation



Observe string encoding of a type representation



Returns a unique integer associated with a TypeRep. This can
be used for making a mapping with TypeReps
as the keys, for example. It is guaranteed that t1 == t2 if and only if
typeRepKey t1 == typeRepKey t2.
It is in the IO monad because the actual value of the key may
vary from run to run of the program. You should only rely on
the equality property, not any actual key value. The relative ordering
of keys has no meaning either.


The other Typeable classes


Note: The general instances are provided for GHC only.



Variant for unary type constructors
  Methods    Instances  



Variant for binary type constructors
  Methods    Instances  



Variant for 3ary type constructors
  Methods    Instances  



Variant for 4ary type constructors
  Methods    Instances  



Variant for 5ary type constructors
  Methods    Instances  



Variant for 6ary type constructors
  Methods    Instances  



Variant for 7ary type constructors
  Methods    Instances  



Cast for * > *



Cast for * > * > *


Default instances


Note: These are not needed by GHC, for which these instances
are generated by general instance declarations.



For defining a Typeable instance from any Typeable1 instance.



For defining a Typeable1 instance from any Typeable2 instance.



For defining a Typeable2 instance from any Typeable3 instance.



For defining a Typeable3 instance from any Typeable4 instance.



For defining a Typeable4 instance from any Typeable5 instance.



For defining a Typeable5 instance from any Typeable6 instance.



For defining a Typeable6 instance from any Typeable7 instance.


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