elm-auto-encoder-decoder

Generate automatic encoder and decoder. Leveraged by choonkeat/elm-webapp Client and Server instances to pass Msg to and fro.

NOTE: While the generated decoders can always decode json encoded by the same version generator, there is no guarantee future versions can decode whatever was encoded in prior versions. :. If you're storing json for later retrieval, I do not recommend using the encoder/decoder here; I don't.

Usage

npx elm-auto-encoder-decoder src/Types.elm

any time src/Types.elm file changes, src/Types/Auto.elm will be regenerated.

ENV variable configurations

GENERATED_SRC will write Auto.elm files to a different directory than the source files, while keeping the directory hierarchy. e.g. GENERATED_SRC=generated/src

WATCHING when set to false will exit after a single run, instead of hanging around waiting for file changes. e.g. WATCHING=false

EXTRA_IMPORT is written to all Auto.elm files if present, e.g. EXTRA_IMPORT='Time.Extra excposing (..)'

How it works

An Elm module Foo.Bar like below

module Foo.Bar exposing (..)

type alias Choice =
    Option Bool


type Option a
    = None
    | Some a

will be encoded and decoded as JSON like

["Foo.Bar.None"]
["Foo.Bar.Some",true]

by the generated Elm functions in the corresponding Foo.Bar.Auto module function

Auto encoder decoders of related types

If src/A.elm imports from src/B.elm, its better to provide both filenames to the cli. The generated encoder/decoder of A will automatically reference the generated encoder/decoder of B

Exposed types

No encoder/decoder functions will be generated for types that are not fully exposed. i.e. a custom type has to be exposed with (..) e.g. MyType(..).

However, if there are exposed types that are composed from unexposed types, you're expected to supply the encoder/decoder functions manually by naming convention (see next point)

Don't be alarmed with "I cannot find ... variable" compiler errors

If your src/Types.elm contain types imported from other modules, e.g. Time.Posix

type alias User = { name : String, createdAt : Time.Posix }

the generated code will likely not compile

-- NAMING ERROR --------------------------------------------- src/Types/Auto.elm

I cannot find a `decodeTimePosix` variable:

248|         |> Json.Decode.map2 (|>) (Json.Decode.at [ "createdAt" ] (decodeTimePosix))
                                                                       ^^^^^^^^^^^^^^^
-- NAMING ERROR --------------------------------------------- src/Types/Auto.elm

I cannot find a `encodeTimePosix` variable:

197|         , ("createdAt", (encodeTimePosix) value.createdAt)
                              ^^^^^^^^^^^^^^^

just implement and expose those function names in your source src/Types.elm (or a different file specified by EXTRA_IMPORT env), for example:

decodeTimePosix : Json.Decode.Decoder Time.Posix
decodeTimePosix =
    Json.Decode.int
        |> Json.Decode.map Time.millisToPosix


encodeTimePosix : Time.Posix -> Json.Encode.Value
encodeTimePosix t =
    Json.Encode.int (Time.posixToMillis t)