[Haskell-beginners] Possible to type a function to a
byorgey at seas.upenn.edu
Sun May 2 10:42:42 EDT 2010
On Sun, May 02, 2010 at 09:54:11AM -0400, Ken Overton wrote:
> Hi fellow beginners (and everyone else),
> As an exercise, I'm implementing a simple, untyped lambda calculus:
> -- a term is a variable, an application, or abstraction (lambda)
> data T = V String | A (T) (T) | L String (T)
> deriving (Eq)
> So I'm writing a function that returns a list of all the free variables in a term and descendants. I can only get it to compile with type:
> freev :: T -> [T]
> It'd be nice for the type of that function to be restricted to just variables like:
> freev :: T -> [V String] -- compile error: "Not in scope: type constructor or class `V'"
> Is there some way to express that? The error seems to suggest maybe haskell could do it if I'd just say it correctly. I mean, isn't "V String" a type constructor?
No, there's no way to do that. V is a data constructor, not a type
constructor; V String is not a type. There's no way to express
"values of such-and-such a type but restricted only to things built
with such-and-such a constructor" without resorting to fancy type
tricks, which you don't really need.
Instead, I would expect freev to have the type
freev :: T -> [String]
It's much more usual to have freev just return the *names* of all the
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