6.12.2. Let-generalisation


Infer less polymorphic types for local bindings by default.

An ML-style language usually generalises the type of any let-bound or where-bound variable, so that it is as polymorphic as possible. With the extension MonoLocalBinds GHC implements a slightly more conservative policy, using the following rules:

  • A variable is closed if and only if
    • the variable is let-bound
    • one of the following holds:
      • the variable has an explicit type signature that has no free type variables, or
      • its binding group is fully generalised (see next bullet)
  • A binding group is fully generalised if and only if
    • each of its free variables is either imported or closed, and
    • the binding is not affected by the monomorphism restriction (Haskell Report, Section 4.5.5)

For example, consider

f x = x + 1
g x = let h y = f y * 2
          k z = z+x
      in  h x + k x

Here f is generalised because it has no free variables; and its binding group is unaffected by the monomorphism restriction; and hence f is closed. The same reasoning applies to g, except that it has one closed free variable, namely f. Similarly h is closed, even though it is not bound at top level, because its only free variable f is closed. But k is not closed, because it mentions x which is not closed (because it is not let-bound).

Notice that a top-level binding that is affected by the monomorphism restriction is not closed, and hence may in turn prevent generalisation of bindings that mention it.

The rationale for this more conservative strategy is given in the papers “Let should not be generalised” and “Modular type inference with local assumptions”, and a related blog post.

The extension MonoLocalBinds is implied by TypeFamilies and GADTs. You can switch it off again with NoMonoLocalBinds but type inference becomes less predictable if you do so. (Read the papers!)