[Haskell-cafe] instance deduction and phantom types

Ryan Ingram ryani.spam at gmail.com
Thu Aug 7 22:51:08 EDT 2008

I'm just guessing from this email; I can't see your code.  Let me know
if I go wrong anywhere.

It looks like you expect your patches to be some type (p :: * -> * ->
*); that is, a type like

> data Patch x y = ...

So I suspect you defined your classes like so:

> class Conflict p where
>     hasConflict :: p x y -> Bool

However, the instance declaration:

> instance Conflict (p x y) => Conflict (PatchInfoAnd (p x y))

is referring to ANY types
   p :: * -> * -> * -> * -> *
   x :: *
   y :: *
so that (p x y :: * -> * -> *); that is, a type like this:

> data PatchBroken a b x y = ...

in this case an instance for Conflict (PatchBroken Int Bool) would
give instances for Conflict (PatchInfoAnd (PatchBroken Int Bool)).

You see, there's no way for the instance declaration for Conflict to
apply from your context (RepoPatch p); in fact, it can't possibly
match, because the "p" in RepoPatch p has kind * -> * -> *, whereas
the "p" in the instance declaration has kind * -> * -> * -> * -> *.

You just had useless instance declarations that would never trigger.
By removing the extra "x y" that were not expected to be there, you
fixed the code to match your actual intent, and the compiler was able
to derive the proper typeclass memberships.

  -- ryan

2008/8/7 Jason Dagit <dagit at codersbase.com>:
> Hello,
> I recently had a problem where GHC couldn't deduce some type classes from my
> constraints.  I would get error messages like:
> Could not deduce (Conflict (PatchInfoAnd p),
>                        Patchy (PatchInfoAnd p))
>        from the context (RepoPatch p)
> But, I had some instances like this (minus the function definitions):
> class (Patchy p, Effect p, FromPrims p, Conflict p) => RepoPatch p
> instance Effect p => Effect (PatchInfoAnd p)
> instance Conflict (p x y) => Conflict (PatchInfoAnd (p x y))
> instance RepoPatch (p x y) => Patchy (PatchInfoAnd (p x y))
> The x and y in the last two are just phantom types on p.  When I noticed
> that the phantom types were missing from the instance definition of Effect
> (PatchInfoAnd p), I removed them from the last two definitions.  This made
> the "Could not deduce..." error message disappear.
> Why do the phantom types not belong in the instance definition?  I'd like to
> understand the problem more fully, but I'm not sure what the problem is or
> how to get started researching it, so if someone could point me in the right
> direction I'd be very happy!
> Thanks!
> Jason
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