constrained-dynamic-0.1.0.0: Dynamic typing with retained constraints

Data.MultiConstrainedDynamic

Description

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

Synopsis

# Types

data ClassConstraint cs Source #

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 MCDynamic cs Source #

A type that contains a value whose type is unknown at compile time, but for which it is known whether or not it satisfies any of a list of constraints, thus allowing operations to be performed when those constraints are satisfied.

Instances

 LTDictSearch css Eq => Eq (MCDynamic css) Source # An instance of Eq for MCDynamic: unwrap the right hand side to the same type as the left (if possible) and if it matches, delegate to the left hand item's instance of Eq if one is available. If either condition fails, return False. Methods(==) :: MCDynamic css -> MCDynamic css -> Bool #(/=) :: MCDynamic css -> MCDynamic css -> Bool # (LTDictSearch css Show, LTDictConstraintLister css) => Show (MCDynamic css) Source # An instance of Show for MCDynamic: delegates to the contained value's definition of showsPrec if an instance of Show is available, or displays the type of the item and its list of available instances otherwise. MethodsshowsPrec :: Int -> MCDynamic css -> ShowS #show :: MCDynamic css -> String #showList :: [MCDynamic css] -> ShowS #

# Functions that mirror functions in Data.Dynamic

toDyn :: (Typeable a, Typeable cs, LTDictBuilder LTDict cs a) => a -> MCDynamic (cs :: [* -> Constraint]) Source #

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

Additionally, there must be appropriate instances of the type class HasClass that describe instances available for type classes that are to be used with the dynamic object and the type to be included in it. In most circumstances you should at least import Data.Type.HasClassPreludeInstances to allow the use of instances of standard Prelude classes and types.

fromDynamic :: Typeable a => MCDynamic css -> Maybe a Source #

Extract a value MCDynamic 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 => MCDynamic cs -> a -> a Source #

Extract a value MCDynamic 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.

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

# Extended API for managing and using class constraints

Return a list of TypeReps that uniquely identify the constraints from an MCDynamics type arguments that are satisfied by the value contained inside the MCDynamic. The actual type whose representation is returned for each constraint is is ClassConstraint c where c is the constraint.

Return a list of TypeReps that uniquely identify the constraints from an MCDynamics type arguments, whether or not they are satisfied by the value contained inside the MCDynamic. The actual type whose representation is returned for each constraint is is ClassConstraint c where c is the constraint.

Return a list of TypeReps that uniquely identify the constraints from an MCDynamics type arguments, that are not satisfied by the value contained inside the MCDynamic. The actual type whose representation is returned for each constraint is is ClassConstraint c where c is the constraint.

applyClassFn :: LTDictSearch css cs => MCDynamic css -> ClassConstraint cs -> (forall a. (cs a, Typeable a) => a -> b) -> Maybe b Source #

Apply a polymorphic function that accepts all values matching a constraint to the value stored inside an MCDynamic wherever possible. If the constraint is satisfied, returns Just r, where r is the result of the function. If the constraint is not satisfied, returns Nothing.

Note that the function *must* be a polymorphic function with only a single argument that is constrained by the constraint, 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.