Martin Foster (aka. EvilTerran) suggested an interesting idea, and I decided it was too nice to ignore/be forgotten inside Martin's head... So I'd like to try and suggest it.

Type wildcards that allow partially specifying types, e.g:

f :: _ -> String
f x = show x

This will instruct the type-inferrer to "fill out" the wild-card part only (e.g: Show a => a).

This feature can be used for a couple of benefits:

A. It allows hard-coding part of a type, rather than the all-or-nothing situation we have now (I want to force a single parameter to be more specific, but I'd still prefer to infer the whole type).

B. Without scoped type variables, there are currently situations you cannot provide a type signature at all.  This may mean that you cannot resolve type ambiguities in some cases, or make types more specific for some purpose.  A type wild-card, in addition to scoped type variables, will allow to only specify the part we want.

C. Its a nice way to kill the -Wall warnings about missing type declarations, if you want to:

f :: _
f = "I am explicitly asking to infer this type, rather than specifying it"

Eyal