ANNOUNCE: GHC 7.0.1 Release Candidate 1
aslatter at gmail.com
Wed Sep 29 22:19:45 EDT 2010
On Wed, Sep 29, 2010 at 9:16 PM, Simon Peyton-Jones
<simonpj at microsoft.com> wrote:
> | 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.
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