elm-codecs

This package allows you to build pairs of JSON encoders (a -> Value) and decoders (Decoder a), collectively called a Codec a.

It supports all of the basic types, collections, records and even custom types! See an example at the bottom of this document.

Design Goals

The design goal is to be as type safe as possible while keeping a nice API. Using this package will greatly reduce the risk of unmatched encoders and decoders.

The packages re-exposes the Value and Decoder types from elm/json, so you don't need to import them too.

Learning Resources

Ask for help on the Elm Slack.

You can also have a look at the FAQ.md file.

Examples

See the examples folder for more examples.

Basic usage

import Codec exposing (Codec, Value)

codec : Codec (List Int)
codec =
    Codec.list Codec.int

encode : List Int -> Value
encode list =
    Codec.encoder codec list

decodeString : String -> Result Codec.Error (List Int)
decodeString s =
    Codec.decodeString codec s

Custom types

type Semaphore
    = Red Int String
    | Yellow Float
    | Green


semaphoreCodec : Codec Semaphore
semaphoreCodec =
    Codec.custom
        (\fred fyellow fgreen value ->
            case value of
                Red i s ->
                    fred i s

                Yellow f ->
                    fyellow f

                Green ->
                    fgreen
        )
        |> Codec.variant2 "Red" Red Codec.int Codec.string
        |> Codec.variant1 "Yellow" Yellow Codec.float
        |> Codec.variant0 "Green" Green
        |> Codec.buildCustom