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]

A description of a custom type error.