FWIW lens has a bit more powerful implementation of Data.Keyed:
https://hackage.haskell.org/package/lens-4.12.3/docs/Control-Lens-At.html#t:...
On Thu, Aug 13, 2015 at 7:18 AM, William Yager
Hello all,
I put together a small library for the purpose of creating overloaded indexing operators. The library is available at https://hackage.haskell.org/package/keyed .
I would like to solicit some advice on the design of this library.
Q1: The library uses TypeFamilies to determine which types to use for the index type (which is used to look up a value) and the value type (which is returned from a lookup).
I originally did this using MultiParamTypeClasses and FunctionalDependencies, but thought this was cleaner; are there any good reasons to go back to using FunDeps?
Q2: Data.Keyed provides pure indexing, while Data.MKeyed provides monadic indexing (e.g. for mutable vectors or concurrent STM-based maps).
I'm having some trouble with the fact that mutable vectors are keyed on the PrimState of their corresponding PrimMonad.
Right now, there is a type in the MKeyed class definition called MContainer. This is the type of the Monad that the lookup operation returns. I.e.
(!) :: MKeyed d => d -> MKey d -> MContainer d (MValue d) ~ (!) :: IOVector a -> Int -> IO a or (!) :: STVector s a -> Int -> ST s a
Unfortunately, this causes an overlap (because both IOVector and STVector are aliased to MVector).
I tried making an instance for MVector, but the problem is that it's difficult to actually go from `IOVector a = MVector RealWorld a` to `IO` or `STVector s a = MVector s a` to `ST`, because neither `IO` nor `ST` appears in the type of the vector. I can't do
instance (PrimMonad m, s ~ PrimState m) => MKeyed (MVector s a) where ... type MContainer (MVector (PrimState m) a) = m
because you can't have type synonyms on the LHS of the type.
I tried bringing `m` into scope using RankNTypes, but that didn't work.
Is there some syntax I can use to bring `m` into scope here?
Or should I be doing this part entirely differently?
Q3: Is there any way to automatically derive all instances of Keyed for all types of Data.Vector (using Data.Vector.Generic)? Using `instance Vector v a => Keyed (v a) where ...` doesn't work, as it overlaps with everything of the form `(d :: * -> *) (a :: *) :: *`, like `[a]`.
Thanks, Will
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