Hi all, This email is literate Haskell. I'm trying to use type families to express some dependencies between type classes, and I'm running into trouble, I think because I'm producing chains of dependencies which the checker can't resolve... Here's a minimised version of the state I've got myself into. :-)
{-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-}
module Families where
First a type family where the type Y is functionally dependent on the type X, and we have a function from Y to ().
class X a where type Y a enact :: Y a -> ()
Now another type family, where the type Q is functionally dependent on the type P, _and_ it must also be an instance of the X class.
class (X (Q s)) => P s where type Q s
(Perhaps there's a better way to express that dependency?) Now a function which takes a value whose type is an instance of the Y depending on the Q depending on the P. (Phew!) The function just tries to call enact on that value.
bar :: P s => Y (Q s) -> () bar w = enact w
The error we get is: src/Families.lhs:35:16: Couldn't match expected type `Y a' against inferred type `Y (Q s)' In the first argument of `enact', namely `w' In the expression: enact w In the definition of `bar': bar w = enact w Presumably this way I'm chaining type dependencies is flawed. Any suggestions on how to improve it, and/or what to read to understand what I'm dealing with better? (So far I've read "Fun with type functions V2", but that's about it, and I admit I didn't grok it all.) Thanks! -Andy