haskell-to-elm-cookbook

[vladimirlogachev]

Here is my idea:

1. Rename this repo to haskell-to-elm-cookbook so that the library users could provide their (potentially opinionated) pairs of Haskell/Elm types, saving time to even more users (but still keep every example fuzz-tested). This will end discussions on how the haskell-to-elm should change, and all of potential "improvements" will end up in the cookbook but not lost.
2. Rearrange the code so that it contains explicitly named modules, e.g. ThingA.hs, ThingA.elm, ThingB.hs, ThingB.elm.

[vladimirlogachev]

E.g. some people will definitely benefit from Set, but it takes some time for a new user to:

1. understand that it's even possible
2. figure out which Elm library to use and how to define Haskell instances

module Common.Set where

import Data.Aeson qualified as Aeson
import Language.Elm.Expression qualified as Expression
import Language.Elm.Type qualified as Type
import Language.Haskell.To.Elm qualified as E

instance E.HasElmType Set where
  elmType =
    "EverySet.EverySet"

instance E.HasElmEncoder Aeson.Value Set where
  elmEncoder =
    "Data.EverySet.encoder"

instance E.HasElmDecoder Aeson.Value Set where
  elmDecoder =
    "Data.EverySet.decoder"

instance (E.HasElmType a) => E.HasElmType (Set a) where
  elmType =
    Type.App "EverySet.EverySet" (E.elmType @a)

instance (E.HasElmEncoder Aeson.Value a) => E.HasElmEncoder Aeson.Value (Set a) where
  elmEncoder =
    Expression.App "Data.EverySet.encoder" (E.elmEncoder @Aeson.Value @a)

instance (E.HasElmDecoder Aeson.Value a) => E.HasElmDecoder Aeson.Value (Set a) where
  elmDecoder =
    Expression.App "Data.EverySet.decoder" (E.elmDecoder @Aeson.Value @a)

module Data.EverySet exposing (decoder, encoder)

import EverySet exposing (EverySet)
import Json.Decode as D
import Json.Encode as E


decoder : D.Decoder a -> D.Decoder (EverySet a)
decoder =
    D.list >> D.map EverySet.fromList


encoder : (a -> E.Value) -> EverySet a -> E.Value
encoder a =
    EverySet.toList >> E.list a

And so on, there are many things to pass back and forth in haskell-to-elm, including popular library types.

[ollef]

I think this is an excellent idea! If you'd like to step up to make this a reality I think it'd be a great addition to the haskell-to-elm ecosystem. With a basic framework in place that might also encourage other contributors to add their own examples.

[vladimirlogachev]

Yeah, I think I am capable of transforming this repo in such way and starting with my use cases. Via PRs, as usual....