On Wed, Sep 29, 2010 at 9:16 PM, Simon Peyton-Jones
| Except I don't know how to write a type signature for this. | | The value 's' passed in is bound by pattern matching on this guy's constructor: | | data Stream a = forall s. Stream (s -> Step s a) !s Int | | in the top-level function, so I don't even know if it has a type I can name.
You can bind the type variable using a pattern type signature
case x of Stream fun (state :: s) n -> ...
This should bind the type variable 's'.
That's the part I'd been missing - I had always thought about type-signatures in patterns as declarations, not about binding a type to a symbol. But that's what the LHS of a match is for, after all. I had been trying all kinds of crazy things. Antoine