[Haskell-beginners] Possible to type a function to a particular
koverton at lab49.com
Sun May 2 09:54:11 EDT 2010
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)
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?
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