Re: Optics?

Hi Alan, hi Vlad, Yes, one thing that is nice about van Laarhoven lenses is that you don't actually need to depend on anything if all you want is export lenses in your API. We have also discussed using a small lens library in the past, in https://gitlab.haskell.org/ghc/ghc/-/issues/18693. The MVP would be to just depend on the Lens module defined in newer versions of Cabal. Sebastian Am Mo., 4. Okt. 2021 um 00:35 Uhr schrieb Alan & Kim Zimmerman < alan.zimm@gmail.com>:

With a pointer from Vlad and some study of the lens tutorial, I made a proof of concept at [1]. I am deliberately not using the existing lens library as I envisage this code ending up in GHC.

Alan

[1] https://github.com/alanz/ghc-exactprint/blob/f218e211c47943c216a2e25d7855f98...

On Sun, 3 Oct 2021 at 18:52, Vladislav Zavialov wrote:

...

Hi Alan,

Your pair of functions can be packaged up as a single function, so that

getEpa :: a -> EpaLocation setEpa :: a -> EpaLocation -> a

becomes

lensEpa :: forall f. Functor f => (EpaLocation -> f EpaLocation) -> (a -> f a)

And the get/set parts can be recovered by instantiating `f` to either Identity or Const.

The nice thing about lenses is that they compose, so that if you need nested access, you could define several lenses, compose them together, and then reach deep into a data structure. Then lenses might offer some simplification. Otherwise, an ordinary getter/setter pair is just as good.

- Vlad

...

On 3 Oct 2021, at 20:40, Alan & Kim Zimmerman wrote:

Hi all

I am working on a variant of the exact printer which updates the annotation locations from the `EpaSpan` version to the `EpaDelta` version, as the printing happens

data EpaLocation = EpaSpan RealSrcSpan | EpaDelta DeltaPos

The function doing the work is this

markAnnKw :: (Monad m, Monoid w) => EpAnn a -> (a -> EpaLocation) -> (a -> EpaLocation -> a) -> AnnKeywordId -> EP w m (EpAnn a)

which gets an annotation, a function to pull a specific location out, and one to update it.

I do not know much about lenses, but have a feeling that I could simplify things by using one.

Can anyone give me any pointers?

Alan

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