coerce is:exact

The function coerce allows you to safely convert between values of types that have the same representation with no run-time overhead. In the simplest case you can use it instead of a newtype constructor, to go from the newtype's concrete type to the abstract type. But it also works in more complicated settings, e.g. converting a list of newtypes to a list of concrete types. When used in conversions involving a newtype wrapper, make sure the newtype constructor is in scope. This function is representation-polymorphic, but the RuntimeRep type argument is marked as Inferred, meaning that it is not available for visible type application. This means the typechecker will accept coerce @Int @Age 42.

Examples

>>> newtype TTL = TTL Int deriving (Eq, Ord, Show)

>>> newtype Age = Age Int deriving (Eq, Ord, Show)

>>> coerce (Age 42) :: TTL
TTL 42

>>> coerce (+ (1 :: Int)) (Age 42) :: TTL
TTL 43

>>> coerce (map (+ (1 :: Int))) [Age 42, Age 24] :: [TTL]
[TTL 43,TTL 25]

coerce v a b coerces v : a to type b, returning a v' : b with maybe extra hidden applications or hidden abstractions. In principle, this function can host coercive subtyping, but currently it only tries to fix problems with hidden function types.

Deprecated: Use coerceInner instead

Coerce items inside a BVar.

Safely coerce between values of types that have the same representation.

If two things are equal you can convert one to the other

If two things are equal, you can convert one to the other.

The function coerce allows you to safely convert between values of types that have the same representation with no run-time overhead. In the simplest case you can use it instead of a newtype constructor, to go from the newtype's concrete type to the abstract type. But it also works in more complicated settings, e.g. converting a list of newtypes to a list of concrete types. This function is runtime-representation polymorphic, but the RuntimeRep type argument is marked as Inferred, meaning that it is not available for visible type application. This means the typechecker will accept coerce @Int @Age 42.