> On Fri, Nov 13, 2009 at 3:26 PM, Andy Gimblett <haskell <at> gimbo.org.uk> wrote: > > 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 -> () > > This is ambiguous. Type families are not injective (that is, Y a ~ Y b > does not imply a ~ b), so there's no way for the compiler to figure > out which instance of X is being used when it encounters enact. > > Given these instances, > > instance X Int where > type Y Int = Bool > enact _ = () > > instance X Char where > type Y Char = Bool > enact _ = undefined > > What is "enact False"? > > I recall seeing a discussion of this in the GHC documentation, but I > can't seem to locate it.

> 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 >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe <at> haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe <at> haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe