[Haskell-cafe] A small (?) problem with type families

Andy Gimblett haskell at gimbo.org.uk
Fri Nov 13 15:26:19 EST 2009

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 (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:

     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.)



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