Hi Ken, You can additionally try to change your definition of lambda-terms to use parametric types as follows. data T var = V var | A (T var) (T var) | L var (T var) Then you can write `freev` with the following type. freev :: T var -> [var] Sincerely, jan. 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?
Thanks,
kov
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