Safe.Coerce

coerce :: forall a b. Coercible a b => a -> b

Coerce a value of one type to a value of some other type, without changing its runtime representation. This function behaves identically to unsafeCoerce at runtime. Unlike unsafeCoerce, it is safe, because the Coercible constraint prevents any use of this function from compiling unless the compiler can prove that the two types have the same runtime representation.

One application for this function is to avoid doing work that you know is a no-op because of newtypes. For example, if you have an Array (Conj a) and you want an Array (Disj a), you could do Data.Array.map (un Conj >>> Disj), but this performs an unnecessary traversal of the array, with O(n) cost. coerce accomplishes the same with only O(1) cost:

mapConjToDisj :: forall a. Array (Conj a) -> Array (Disj a)
mapConjToDisj = coerce

class Coercible (a :: k) (b :: k) 

Coercible is a two-parameter type class that has instances for types a and b if the compiler can infer that they have the same representation. Coercible constraints are solved according to the following rules:

• reflexivity, any type has the same representation as itself: Coercible a a holds.

• symmetry, if a type a can be coerced to some other type b, then b can also be coerced back to a: Coercible a b implies Coercible b a.

• transitivity, if a type a can be coerced to some other type b which can be coerced to some other type c, then a can also be coerced to c: Coercible a b and Coercible b c imply Coercible a c.

• Newtypes can be freely wrapped and unwrapped when their constructor is in scope:

  newtype Age = Age Int
  

Coercible Int Age and Coercible Age Int hold since Age has the same runtime representation than Int.

Newtype constructors have to be in scope to preserve abstraction. It's common to declare a newtype to encode some invariants (non emptiness of arrays with Data.Array.NonEmpty.NonEmptyArray for example), hide its constructor and export smart constructors instead. Without this restriction, the guarantees provided by such newtypes would be void.

• If none of the above are applicable, two types of kind Type may be coercible, but only if their heads are the same. For example, Coercible (Maybe a) (Either a b) does not hold because Maybe and Either are different. Those types don't share a common runtime representation so coercing between them would be unsafe. In addition their arguments may need to be identical or coercible, depending on the roles of the head's type parameters. Roles are documented in the PureScript language reference.

Coercible being polykinded, we can also coerce more than types of kind Type:

• Rows are coercible when they have the same labels, when the corresponding pairs of types are coercible and when their tails are coercible: Coercible ( label :: a | r ) ( label :: b | s ) holds when Coercible a b and Coercible r s do. Closed rows cannot be coerced to open rows.

• Higher kinded types are coercible if they are coercible when fully saturated: Coercible (f :: _ -> Type) (g :: _ -> Type) holds when Coercible (f a) (g a) does.

This rule may seem puzzling since there is no term of type _ -> Type to apply coerce to, but it is necessary when coercing types with higher kinded parameters.