Re: [Haskell-cafe] What does the `forall` mean ?

Eugene Kirpichov wrote:

2009/11/12 Andrew Coppin :

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Even I am still not 100% sure how placing forall in different positions does different things. But usually it's not something I need to worry about. :-)

To me it does not look like it does different things: everywhere it denotes universal polymorphism. What do you mean? I might be missing something.

I think what he means is that this: foo :: forall a b. (a -> a) -> b -> b uses ScopedTypeVariables, and introduces the type-name a to be available in the where clause of myid. Whereas something like this: foo2 :: (forall a. a -> a) -> b -> b uses Rank2Types (I think?) to describe a function parameter that works for all types a. So although the general concept is the same, they use different Haskell extensions, and one is a significant extension to the type system while the other (ScopedTypeVariables) is just some more descriptive convenience. Thanks, Neil.