EscapedSkolem Error

Example

test = do
  r <- runST (newSTRef 0)
  return 0

Cause

Skolem constants are types which only unify with themselves, and a limited range of unification variables. They are used to check polymorphic types, usually when using Rank N Types.

We say that a skolem escapes its scope when an attempt is made to unify it with a unification variable which was generated outside the scope in which the skolem was generated.

In the example above, the type of runST causes us to check newSTRef 0 against a polymorphic type, unifying the type of r with a skolem constant. However, the type of r is fresh, generated outside the scope of newSTRef 0, so we see an EscapedSkolem error. This is good, because we don't want the reference to leak outside of runST.

Fix

• If you are using the ST effect, you might be leaking a reference to a mutable variable. Carefully inspect the types of any values which might leak outside the call to runST, and make sure this is not the case.

Notes

$ and runST

One common pitfall is to use the runST or runPure functions with the $ operator. This will often lead to the EscapedSkolem error due to instantiation of type variables. Avoid this by using parentheses instead of $.

Point free style

In some cases point free style can also leak type variables.

Impredicative Types

You can also encounter this error when trying to use impredicative polymorphism, e.g. when a type variable should be instantiated with a polymorphic type. An example could be using a polymorphic type in a data structure, or defining a type alias to a universally quantified type and using it as a type parameter. See GHC docs or this tutorial to learn more about the problem.