SomeDictOf

Documentation

type SomeDict = SomeDictOf Proxy

In the case where you only need evidence that the dictionary exists, then you can use this type. By carrying a Proxy instead of a real value, we can summon these up whenever we have an instance of the type in question.

Since: 0.1.0.0

data SomeDictOf (f :: k -> Type) (c :: k -> Constraint) where

A datatype that carries evidence of type class membership for some type, along with a datatype indexed by that type. This is confusing, so let's look at some examples.

The alias SomeDict clazz uses Proxy for the datatype index. This means that the wrapper is a Proxy :: Proxy a, and you know that the type a has an instance of clazz a. We can construct a value

showSomeDict :: SomeDictOf Identity Show
showSomeDict =
    SomeDict (Identity 3 :: Identity Int)

A value of SomeDict Show contains Proxy value. When you pattern match on the SomeDict, you know that the given Proxy :: Proxy a has an instance of Show.

For the most part, this isn't useful,

Since: 0.1.0.0

Constructors

SomeDictOf :: c a => f a -> SomeDictOf f c

someDict :: forall a c. c a => SomeDict c

Construct a SomeDict with the type in question. Use with TypeApplications.

Example:

showDict :: SomeDict Show
showDict = someDict Int

entityDict :: SomeDict PersistEntity
entityDict = someDict User

Since: 0.1.0.0

fromDict :: forall a c. Dict (c a) -> SomeDict c

Construct a SomeDict based on a Dict from the Data.Constraint module.

Since: 0.1.0.0

withSomeDictOf :: SomeDictOf f c -> (forall a. c a => f a -> r) -> r

Unpack a SomeDictOf and attain access to the evidence that the underlying type satisfies the instance in question.

Since: 0.1.0.0

mapSomeDictOf :: (forall a. c a => f a -> g a) -> SomeDictOf f c -> SomeDictOf g c

Transform the type index used in the SomeDictOf. The provided function is unable to inspect the type, but you will have the class operations available to it.

Example:

provideMempty ;: SomeDictOf Proxy Monoid -> SomeDictOf Identity Monoid
provideMempty = mapSomeDictOf (\proxy -> Identity mempty)

Since: 0.1.0.0

forgetContents :: forall f c. Functor f => SomeDictOf f c -> f (SomeDictOf Proxy c)

Not really sure what this might be useful for.

Examples:

shows :: SomeDictOf [] Show
shows = SomeDictOf [True, False, True]

forgotten :: [SomeDictOf Proxy Show]
forgotten = forgetContents show

main = do
    forM forgotten $ \(SomeDictOf Proxy) -> do
         print 10

The above program should output 10 three times.

Since: 0.1.0.0

Re-exports

module Data.Functor.Identity

module Data.Proxy

module Data.Constraint