[Haskell-cafe] A voyage of undiscovery

wren ng thornton wren at freegeek.org
Sat Jul 18 19:52:48 EDT 2009

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 

     Robin Milner. A theory of type polymorphism in programming.
         Journal of Computer and System Sciences, 17:348-375, August

Live well,

More information about the Haskell-Cafe mailing list