MPTCs and functional dependencies
ravi at bluespec.com
Thu Feb 2 09:26:28 EST 2006
Henrik Nilsson wrote:
> Dear all,
> Simon PJ wrote:
> > Multi-parameter type classes, yes. Functional dependencies, no.
> My experience is that even with very simple applications of MPTCs,
> I often end up needing functional dependencies to make things work.
As a user, I'll echo this. It seems to me that when you create a
multi-parameter typeclass, there are often constraints among the
different parameters that need to be expressed. As an example, consider
writing a multi-parameter typeclass to capture the idea of being able to
"select" an object out of an indexed collection (e.g. a list or an
array). One dependency you'd like to have might be that the collection
type determines the type of the item you're selecting (so that the type
of the item can be inferred from the type of the collection).
As a second data point, I looked at the monad transformer library. The
multi-parameter classes MonadState, MonadError, MonadReader and
MonadWriter all use functional dependencies among their parameters. I
think monad transformers are a significant part of the motivation to
include multi-parameter typeclasses. This, to me, implies that we need
some answer for their use of functional dependencies.
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