created_at2022-02-03 00:33:32.284729
updated_at2023-12-09 19:10:52.049618
descriptionGenerate Elm bindings for your Rust types
Martinez (Heliozoa)




Automatically generate type definitions and functions for your Elm frontend from your Rust backend types, making it easy to keep the two in sync. Currently supports generating

  • Elm types with the Elm trait and derive macro
  • JSON encoders with the ElmEncode trait and derive macro, compatible with serde_json
  • JSON decoders with the ElmDecode trait and derive macro, compatible with serde_json
  • URL query encoders with the ElmQuery and ElmQueryField traits and derive macros


For example, the following code

use elm_rs::{Elm, ElmEncode, ElmDecode, ElmQuery, ElmQueryField};

#[derive(Elm, ElmEncode, ElmDecode)]
enum Filetype {

#[derive(Elm, ElmEncode, ElmDecode)]
struct Drawing {
    title: String,
    authors: Vec<String>,
    filename: String,
    filetype: Filetype,

#[derive(Elm, ElmQuery)]
struct Query {
    page: usize,
    thumbnail_size: Size,

#[derive(Elm, ElmQueryField)]
enum Size {

fn main() {
    // the target would typically be a file
    let mut target = vec![];
    // elm_rs provides a macro for conveniently creating an Elm module with everything needed
    elm_rs::export!("Bindings", &mut target, {
        // generates types and encoders for types implementing ElmEncoder
        encoders: [Filetype, Drawing],
        // generates types and decoders for types implementing ElmDecoder
        decoders: [Filetype, Drawing],
        // generates types and functions for forming queries for types implementing ElmQuery
        queries: [Query],
        // generates types and functions for forming queries for types implementing ElmQueryField
        query_fields: [Size],
    let output = String::from_utf8(target).unwrap();
    println!("{}", output);

prints out

-- generated by elm_rs

module Bindings exposing (..)

import Dict exposing (Dict)
import Http
import Json.Decode
import Json.Encode
import Url.Builder

resultEncoder : (e -> Json.Encode.Value) -> (t -> Json.Encode.Value) -> (Result e t -> Json.Encode.Value)
resultEncoder errEncoder okEncoder enum =
    case enum of
        Ok inner ->
            Json.Encode.object [ ( "Ok", okEncoder inner ) ]
        Err inner ->
            Json.Encode.object [ ( "Err", errEncoder inner ) ]

resultDecoder : Json.Decode.Decoder e -> Json.Decode.Decoder t -> Json.Decode.Decoder (Result e t)
resultDecoder errDecoder okDecoder =
        [ Json.Decode.map Ok (Json.Decode.field "Ok" okDecoder)
        , Json.Decode.map Err (Json.Decode.field "Err" errDecoder)

type Filetype
    = Jpeg
    | Png

filetypeEncoder : Filetype -> Json.Encode.Value
filetypeEncoder enum =
    case enum of
        Jpeg ->
            Json.Encode.string "Jpeg"
        Png ->
            Json.Encode.string "Png"

type alias Drawing =
    { title : String
    , authors : List (String)
    , filename : String
    , filetype : Filetype

drawingEncoder : Drawing -> Json.Encode.Value
drawingEncoder struct =
        [ ( "title", (Json.Encode.string) struct.title )
        , ( "authors", (Json.Encode.list (Json.Encode.string)) struct.authors )
        , ( "filename", (Json.Encode.string) struct.filename )
        , ( "filetype", (filetypeEncoder) struct.filetype )

filetypeDecoder : Json.Decode.Decoder Filetype
filetypeDecoder = 
        [ Json.Decode.string
            |> Json.Decode.andThen
                (\x ->
                    case x of
                        "Jpeg" ->
                            Json.Decode.succeed Jpeg
                        unexpected ->
                            Json.Decode.fail <| "Unexpected variant " ++ unexpected
        , Json.Decode.string
            |> Json.Decode.andThen
                (\x ->
                    case x of
                        "Png" ->
                            Json.Decode.succeed Png
                        unexpected ->
                            Json.Decode.fail <| "Unexpected variant " ++ unexpected

drawingDecoder : Json.Decode.Decoder Drawing
drawingDecoder =
    Json.Decode.succeed Drawing
        |> Json.Decode.andThen (\x -> Json.Decode.map x (Json.Decode.field "title" (Json.Decode.string)))
        |> Json.Decode.andThen (\x -> Json.Decode.map x (Json.Decode.field "authors" (Json.Decode.list (Json.Decode.string))))
        |> Json.Decode.andThen (\x -> Json.Decode.map x (Json.Decode.field "filename" (Json.Decode.string)))
        |> Json.Decode.andThen (\x -> Json.Decode.map x (Json.Decode.field "filetype" (filetypeDecoder)))

type alias Query =
    { page : Int
    , thumbnailSize : Size

urlEncodeQuery : Query -> List Url.Builder.QueryParameter
urlEncodeQuery struct =
    [ Url.Builder.int "page" (identity struct.page), Url.Builder.string "thumbnail_size" (queryFieldEncoderSize struct.thumbnailSize) ]

type Size
    = Small
    | Large

queryFieldEncoderSize : Size -> String
queryFieldEncoderSize var =
    case var of
        Small -> "Small"
        Large -> "Large"


Cargo features

  • derive: Activated by default. Enables deriving the Elm and ElmEncode traits.
  • serde: Enables compatibility with many of serde's attributes. (serde v1)
  • chrono: Trait implementations for chrono types. (chrono v0.4)
  • time: Trait implementations for time types. (time v0.3)
  • uuid: Trait implementations for uuid types. (uuid v1)

Serde compatibility

The serde feature enables compatibility with serde attributes. Currently the following attributes are supported:

Container attributes

  • rename_all
  • tag
  • tag & content
  • untagged
  • transparent

Variant attributes

  • rename
  • rename_all
  • skip
  • other

Field attributes

  • rename
  • skip


  • Generate Elm types with the Elm trait and derive macro
  • Generate JSON encoders and decoders with the ElmEncode and ElmDecode traits and derive macros
  • Basic generic support
  • Compatibility with most serde attributes
  • Support for simple queries


  • Support for forms and complex queries
  • Compatibility with more serde attributes
    • flatten
    • alias
    • skip_(de)serializing
  • Optionally include definitions for the dependencies of exported types
  • Implement support for more serde::{Deserialize, Serialize} std types
    • IpAddr, Ipv4Addr, Ipv6Addr
    • SocketAddr, SocketAddrV4, SocketAddrV6
    • PhantomData
  • Handle recursive types
  • Attributes for controlling the name of the Elm type etc.

Known limitations

Generic types are not well supported when they are used with more than one set of concrete types. For example, for

struct Generic<T>(T);

Generic::<u32>::elm_definition() and Generic::<String>::elm_definition() will both use the name Generic for the Elm definition, causing an error in Elm. Accidentally using different generic types for the generated JSON and the Elm definition can also result in some confusing error messages.

Reusing enum variant names is allowed in Rust but not in Elm. Therefore generating definitions for the two enums

enum Enum1 {
enum Enum2 {

will cause an error in Elm due to Variant being ambiguous.



Licensed under Mozilla Public License Version 2.0

Commit count: 67

cargo fmt