[Haskell-cafe] Library for overloaded indexing (a la (!))

[Patrick Chilton]

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:Ixed

On Thu, Aug 13, 2015 at 7:18 AM, William Yager <will.yager at gmail.com> wrote:

> 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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