Probably unrelated, but this thread is what triggered it for me. There is a minor bug in showing impredicative types without -fglasgow-exts: *hope I got that right*
Prelude> let x = [] :: [forall a. a]
<interactive>:1:23: Warning: Accepting non-standard infix type constructor `.' Use -fglasgow-exts to avoid this warning Prelude> :t x x :: [. (forall a) a]
^^^^^^^^^^^^^^^^
This is a minor bug, but not the one I think you mean. Look at the message. GHC says it's accepting the non-standard infix type constructor called `.' ... So it's really interpreting `.' as an infix type variable. It is also interpreting `forall' as a type variable, because without -fglasgow-exts, `forall' is not a keyword, so it's a valid variable name. So, renaming the `.' to `x' and `forall' to `f', your expression is equivalent to Prelude> let x = [] :: [x (f a) a] and that is type-correct. This is also very close to what GHC prints as the type, namely [. (forall a) a] So GHC decides to put `.' in prefix position, which is ok also for infix type operators, but you have to put them in parentheses for GHC to re-accept the type, so the bug is that GHC should really print [(.) (forall a) a] as type of `x'. Cheers, Andres