Friday, November 13, 2020

Elm Codegen Quirks with Servant-Elm and Persistent

Developing further on the previous post, building some slightly less trivial examples with the combination of servant-elm and persistent (with Sqlite backend) yields some interesting problems in the generated Elm code. 

1. Phantom Types for Keys

Due to the way persistent handles database primary keys, servant-elm (via the underlying elm-bridge) currently generates Elm code with types such as Key World instead of WorldId, where, for Sqlite, ID variables are of type Int, used for database primary keys. 

But what is Key World in terms of Elm types? And what happens when you try to sequence a JSON encoder / decoder for Key (which doesn't exist!), along with a JSON encoder / ecoder for World -- something altogether different from the desired Int

type alias User  =
   { userEnv: (Key World)
   , userName: String


jsonEncUser : User -> Value
jsonEncUser  val =
   [ ("userEnv", (jsonEncKey (jsonEncWorld)) val.userEnv)
   , ("userName", Json.Encode.string val.userName)

There's some discussion of handling phantom types in elm-bridge, as well as some existing support for custom type replacements in the elm-bridge libraryTo get it working, I ended up needing the custom type replacements, as well as some direct manipulation of the generated Elm code. (I also found some fairly extensive replacement ideas here).

That takes care of the instances of Key World and the like in Elm type definitions, but not in the API call functions, and also introduces a new problem of missing definitions for Id type encoders/decoders. 

type alias User  =
   { userEnv: WorldId
   , userName: String

jsonEncUser : User -> Value
jsonEncUser  val =
   [ ("userEnv", jsonEncWorldId val.userEnv)
   , ("userName", Json.Encode.string val.userName)

getWorldByWid : (Key World) -> (Result Http.Error  ((Maybe World))  -> msg) -> Cmd msg
getWorldByWid capture_wid toMsg =

There may be some way to do replacements in a more integral way with the Template Haskell codegen, but I ended up just doing appends and naive text replacements on the final Elm output:

2. Missing URL String Encoders

I also couldn't get servant-elm to generate correct URL string encoders, despite defining instances of toHttpApiData, which seems like it should be the thing to do

            , url =
                Url.Builder.crossOrigin "http://localhost:8080"
                    [ "cellColor"
                    , (capture_worldid |> String.fromInt)
                    , (capture_x |> String.fromInt)
                    , (capture_y |> String.fromInt)
                    , (capture_color |> String.fromInt)

... where Color is defined on the Haskell side as:

data Color
  = White
  | Yellow
  | Red
  | Green
  | Blue
  | Grey
  deriving (Show, Read, Eq, Generic, Enum, Bounded)

instance FromHttpApiData Color where
  parseUrlPiece :: T.Text -> Either T.Text Color
  parseUrlPiece = myParse

instance ToHttpApiData Color where
  toUrlPiece = T.pack . show
  toQueryParam = T.pack . show

So for each sum type instance, the post-code-gen script from above also takes care of the string encoders for use in URL building, appending the functions to the generated Elm code.


Wrapping Up

With the above edits, the Elm code gen + persistence layer stack works again. There was a lot more research and work(arounds) required this time, and I needed some help from friends at the Recurse Center, but it still (mostly!) satisfies the property of writing data definitions only once for the whole stack. It's not hard to see that some of the type mapping issues across languages and data boundaries are non-trivial to resolve, so I hope more usage and development goes into these kinds of type-safe stacks. 

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