[Haskell-cafe] Proposal for GHC: global HasFoo classes

[Silvio Frischknecht]

I don't have the time to flesh out everything, but I thought this kind
of problem can usually be solved with Symbols.

https://hackage.haskell.org/package/base-4.10.1.0/docs/GHC-TypeLits.html

Cheers

Silvio


On 13.02.2018 05:39, KAction at gnu.org wrote:
> Hello!
>
> I have idea for GHC to make working with lenses even more convenient.
>
> # Rationale
>
> Let us recall, that "type Lens' a b = (a -> f a) -> (b -> f b)" is about
> accessing some value of type `a' somewhere deep in value of type `b'.
>
> Let us consider that we have following datatypes:
>
>   -- module 1
>   {-# LANGUAGE DisambiguateRecordFields  #-}
>   data Foo = Foo { _a :: Int, _b :: Double }
>   data Bar = Bar { _a :: Double, _c :: Char }
>
> Since we want to be able to use 'a' as both lens into 'Foo' and into
> 'Bar', we would use 'makeFields' function from 'lens' package, and get
> something like this generated (implementation is omited):
>
>   -- module 1
>   class HasA w p | w -> p where a :: Lens' w p
>   class HasB w p | w -> p where b :: Lens' w p
>   class HasC w p | w -> p where c :: Lens' w p
>   instance HasA Foo Int
>   instance HasA Bar Double
>   instance HasB Foo Double
>   instance HasC Bar Char
>
> It is all great and convenient, but what if we have another module with
>
>   -- module 2
>   data Quiz = Quiz { _a ::  Bool }
>
> In same spirit, 'makeFields' will create
>
>   -- module 2
>   class HasA w p | w -> p where a :: Lens' w p
>   instance HasA Quiz Bool
>
> Now, you import unqualified both modules, and you have two versions of
> 'a' function, one per module. From GHC's point of view, class HasA in
> module 1 and module 2 are different, while we, humans, understand, that
> they are not.
>
> To mitigate this problem, common convention is to collect all types,
> used in package, and their lenses in signle module. But there is no
> solution {as far as I know} for lenses from different packages.
>
> # Proposal
>
> Let us introduce a new extension -XGlobalLensClasses (better name is
> welcome). When this extension is activated in module, you can write
> instances like
>
>   instance HasFoo Foo Int where foo = -- apporiate lens
>
> with following implicit class definition:
>
>   class HasFoo w p | w -> p where foo :: Lens' w p
>
> After that, module can export 'foo'. All implicit HasFoo classes, whose
> instances are defined in different modules are considered the same. It
> eliminates problem, described in rationale if both modules in
> consideration used proposed extension.
>
> It should be noted, that this extension would not disrupt any existing
> code, although in distant future we could have it by default.
>
> # Alternative idea
>
> The proposal of making global only one particular class namespace
> 'Has<Foo>' with very specific signature is not generic enough.  Maybe we
> can add option to mark class definition as global instead, merging all
> global classes with same signature together?
>
> Sounds more complicated, but this solution will not upset those people
> who disagree with Lens-based HasFoo classes.
>
> Opinions?
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.