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@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.
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