Ack. I've just realised that P/Q is not a functional dependency. I
need to use a multi-parameter type class there. So my question is
probably completely pointless - sorry!
Thanks anyway,
-Andy
On 13 Nov 2009, at 20:26, Andy Gimblett wrote:
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
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