Andrew Coppin wrote:
Robert Greayer wrote:
f0 _ = (foo True, foo 'x') where foo = id
is well-typed.
Really? That actually works? How interesting... This suggests to me that where-clauses also do strange things to the type system.
Not too strange, in fact we need it to do that for local definitions to be helpful at all. The short answer is that let-binding is still polymorphic, whereas lambda-binding (passing in a parameter) is monomorphic. If let-binding were not polymorphic, then we could remove it entirely can just desugar everything into lambda-bindings. You should read this classic paper which introduced Hindley--Milner type inference, Robin Milner. A theory of type polymorphism in programming. Journal of Computer and System Sciences, 17:348-375, August 1978. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.67.5276 -- Live well, ~wren