[Haskell-cafe] Is "let" special?

[Ryan Ingram]

2010/11/3 Petr Pudlak <deb at pudlak.name>:
> However, from the typing point of view, it makes quite a difference. It is
> an integral part of the Hindley-Milner type inference algorithm, which is
> used by many functional languages. Consider the following two expressions:
>
>> f = (\x -> x x) (\y -> y)
>> g = let x = \y -> y in x x
>
> The function "f" is not typable in the Hindley-Milner type system, while "g"
> is is (and its type is "a -> a"). The reason is that in the first case (f),
> the typing system tries to assign a single type to "x", which is impossible
> (one would have to unify types "a" and "a -> b"). In the second case (g),
> the typing system assigns "x" a polymorphic type schema
>  forall a.a -> a
> which can be instantiated to both "a -> a" (the second "x") and "(b -> b) ->
> (b -> b)" (the first "x"), and then applied together. I'd say that "let ...
> in ..." is a core feature of the language that gives us parametric
> polymorphism.

Although this decision is not without problems.  See the recent paper
"Let should not be generalised" [1]

  -- ryan

[1] Let should not be generalised, Dimitrios Vytiniotis, Simon Peyton
Jones, and Tom Schrijvers; submitted to TLDI 2010.
http://research.microsoft.com/en-us/um/people/simonpj/papers/constraints/index.htm