Eugene Kirpichov wrote:
2009/11/12 Andrew Coppin <andrewcop...@btinternet.com>:
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.
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