On Tue, 2008-06-10 at 18:12 +0200, Roberto Zunino wrote:
Sean Leather wrote:
inside :: ((forall a. W (t a))-> W (forall a. (t a))) --inside (W x) = W x -- (a) FAILS --inside = W . unW -- (b) FAILS inside x = W (unW x) -- (c) WORKS
Are there any pointers for developing a better understanding or intuition of this?
Usually, making type arguments explicit helps: that is, assume that each polymorphic value is actually a function expecting a type (the a in forall a. ...) and returning the value related to that type. The last 'inside' definition becomes:
inside x = W (\type -> unW (x type))
Note that we do not know at this time what will be the actual "type" above. But that's OK, since we can return a type-lambda.
Instead, pattern matching desugars to:
inside x = case x of ...
But wait! x is a function (because it's polymorphic) and we can not pattern match on that! And, at this time, we do not know the type-argument for x. So we are stuck.
Also, type-lambdas and type-arguments are hard to insert in W . unW .
This is the lack of impredicativity. W :: a -> W a To get the result type W (forall a. t a), W must instantiate the a in W's type to (forall a. t a). Further we then pass it to (.) which has type (b -> c) -> (a -> b) -> a -> c and thus require instantiating both a and b to higher-rank types. A predicative type system does not allow instantiating type variables to quantified types.