Haskell: Are type variables in “where” clauses in the same namespace with their parents?

Question

In the following snippet (I have abstracted all other trivial parts)

data T s = T (s -> s)

foo :: T s -> s -> s
foo (T f) x = bar x where
    bar :         


        

Answer 1

ScopedTypeVariables are indeed the solution, but there's an extra requirement to use them: You must put explicit foralls in the type signatures declaring the variables you want to scope, like this:

foo :: forall s. T s -> s -> s

This is so that the meaning of signatures in code doesn't depend on whether the extension is enabled or not.