Data.ConstrainedDynamic

Provides a container type similar to Data.Dynamic but which retains information about a typeclass (or other constraint) that is known to be available for the type of the object contained inside.

Types

data ClassConstraint cs

A type used to represent class constraints as values. This exists primarily so that typeOf (ClassConstraint :: ClassConstraint cs) can be used to obtain a TypeRep that uniquely identifies a typeclass.

Constructors

ClassConstraint

data ConstrainedDynamic cs

A type that contains a value whose type is unknown at compile time, except that it satisfies a given constraint. For example, a value of ConstrainedDynamic Show could contain a value of any type for which an instance of the typeclass Show is available.

Instances

Typeable (* -> Constraint) cs => Show (ConstrainedDynamic cs)

An instance of Show for ConstrainedDynamic: delegates to the contained value's definition of showsPrec if the constraint is Show, or shows the type of the contained value otherwise.

Methods

showsPrec :: Int -> ConstrainedDynamic cs -> ShowS

show :: ConstrainedDynamic cs -> String

showList :: [ConstrainedDynamic cs] -> ShowS

Functions that mirror functions in Data.Dynamic

toDyn :: (Typeable a, cs a, Typeable cs) => a -> ConstrainedDynamic cs

Create a ConstrainedDynamic for a given value. Note that this function must be used in a context where the required constraint type can be determined, for example by explicitly identifying the required type using the form toDyn value :: ConstrainedDynamic TypeClass.

fromDynamic :: (Typeable a, cs a) => ConstrainedDynamic cs -> Maybe a

Extract a value ConstrainedDynamic to a particular type if and only if the value contained with in it has that type, returning Just v if the value v has the correct type or Nothing otherwise,

fromDyn :: (Typeable a, cs a) => ConstrainedDynamic cs -> a -> a

Extract a value ConstrainedDynamic to a particular type if and only if the value contained with in it has that type, returning the value if it has the correct type or a default value otherwise.

dynTypeRep :: ConstrainedDynamic cs -> TypeRep

Return the TypeRep for the type of value contained within a ConstrainedDynamic.

Extended API for managing and using class constraints

dynConstraintType :: forall a. Typeable a => ConstrainedDynamic a -> TypeRep

Return a TypeRep that uniquely identifies the type of constraint used in the ConstrainedDynamic. The actual type whose representation is returned is ClassConstraint c where c is the constraint.

applyClassFn :: ConstrainedDynamic cs -> (forall a. cs a => a -> b) -> b

Apply a polymorphic function that accepts all values matching the appropriate constraint to the value stored inside a ConstrainedDynamic and return its result. Note that this *must* be a polymorphic function with only a single argument that is constrained by the constrain, so for example the function show from the typeclass Show is allowable, but == from the typeclass Eq would not work as it requires a second argument that has the same type as the first, and it is not possible to safely return the partially-applied function as its type is not known in the calling context.

classCast :: forall a b. (Typeable a, Typeable b) => ConstrainedDynamic a -> Maybe (ConstrainedDynamic b)

If a ConstrainedDynamic has an unknown constraint variable, classCast can be used to convert it to a ConstrainedDynamic with a known constraint. For example, classCast d :: Maybe (ConstrainedDynamic Show) returns Just d :: Maybe (ConstrainedDynamic Show) if ds constraint was Show or Nothing if it was any other constraint.