[Haskell-cafe] A voyage of undiscovery

[wren ng thornton]

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