Safe Haskell | None |
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# Documentation

The datatype of Keys.

- x
- The scope of this key. This can either be
`T`

for top-level keys created with`createKey`

or an existential type for keys introduced by`withKey`

(or with the Key monad`KeyM`

). - a
- The type of things that can be sorted at this key.

For example, `Key T Int`

is a top-level key that can be used to store values
of type `Int`

in a heterogenous map.

withKey :: (forall x. HKey x a -> b) -> bSource

*O(1)*. Scopes a key to the given function
The key cannot escape the function (because of the existential type).

The implementation actually *creates* a key, but because the key cannot escape
the given function `f`

, there is no way to observe that if we run
`withKey f`

twice, that it will get a different key the second time.