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 <syntaxgli...@gmail.com> > On Sat, Apr 17, 2010 at 4:01 PM, Limestraël <limestr...@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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