[Haskell-cafe] Functional Dependencies conflicts

Limestraël limestrael at gmail.com
Sat Apr 17 17:11:50 EDT 2010

Ok, so I am heading to a headache...

Daniel Fischer mentioned a solution using Type Families. As I read, those
are meant to replace the FunDeps, I will try this solution...

2010/4/17 Casey McCann <syntaxglitch at gmail.com>

> On Sat, Apr 17, 2010 at 4:01 PM, Limestraël <limestrael at gmail.com> wrote:
> > I would have undestood the error if GameObject was also an instance of
> > Binary (then the two instances would match), but it's not the case...
> As Daniel Fischer has mentioned, presumably a Binary instance could
> later be written for GameObject; even if you have no intention of
> doing so, GHC considers the possibility. In other words, it's
> sufficient that such an instance could exist, not that it actually
> does.
> In general: Instance selection and context checking are separate and
> occur in that order, thus contexts generally can't influence instance
> selection (except by using OverlappingInstances and strategically
> confusing instance heads, ensuring that GHC can't make any sense of
> your code until considering the contexts).
> Unfortunately, anything involving extremely generic instances with
> some constraint tend to be very difficult to construct, because of how
> this works. This tends to include things like default instances for
> types not covered by specific ones, making all instances of X also
> instances of Y, fundep "type predicates" based on class membership,
> and so on. Type hackery can often get you most of what you want, but
> it gets awkward fast.
> - C.
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