A value that knows how to encode and decode JSON values.

Represents a JavaScript value.

A structured error describing exactly how the decoder failed. You can use this to create more elaborate visualizations of a decoder problem. For example, you could show the entire JSON object and show the part causing the failure in red.

A value that knows how to decode JSON values.

Extracts the Decoder contained inside the Codec.

Parse the given string into a JSON value and then run the Codec on it. This will fail if the string is not well-formed JSON or if the Codec fails for some reason.

Run a Codec to decode some JSON Value. You can send these JSON values through ports, so that is probably the main time you would use this function.

Extracts the encoding function contained inside the Codec.

Convert a value into a prettified JSON string. The first argument specifies the amount of indentation in the result string.

Convert a value into a Javascript Value.

Codec between a JSON string and an Elm String

Codec between a JSON boolean and an Elm Bool

Codec between a JSON number and an Elm Int

Codec between a JSON number and an Elm Float

Codec between a JSON string of length 1 and an Elm Char

Represents an optional value.

Codec between a JSON array and an Elm List.

Codec between a JSON array and an Elm Array.

Codec between a JSON object and an Elm Dict.

Codec between a JSON array and an Elm Set.

Codec between a JSON array of length 2 and an Elm Tuple.

Codec between a JSON array of length 3 and an Elm triple.

Codec for Result values.

A partially built Codec for an object.

Start creating a Codec for an object. You should pass the main constructor as argument. If you don't have one (for example it's a simple type with no name), you should pass a function that given the field values builds an object.

Example with constructor:

type alias Point =
    { x : Float
    , y : Float
    }


pointCodec : Codec Point
pointCodec =
    Codec.object Point
        |> Codec.field "x" .x Codec.float
        |> Codec.field "y" .y Codec.float
        |> Codec.buildObject

Example without constructor:

pointCodec : Codec { x : Int, y : Bool }
pointCodec =
    Codec.object (\x y -> { x = x, y = y })
        |> Codec.field "x" .x Codec.int
        |> Codec.field "y" .y Codec.bool
        |> Codec.buildObject

Specify the name, getter and Codec for a field.

The name is only used as the field name in the resulting JSON, and has no impact on the Elm side.

Specify the name getter and Codec for an optional field.

This is particularly useful for evolving your Codecs.

If the field is not present in the input then it gets decoded to Nothing. If the optional field's value is Nothing then the resulting object will not contain that field.

Specify the name getter and Codec for a required field, whose value can be null.

If the field is not present in the input then the decoding fails. If the field's value is Nothing then the resulting object will contain the field with a null value.

This is a shorthand for a field having a codec built using Codec.maybe.

Create a Codec from a fully specified ObjectCodec.

A partially built Codec for a custom type.

Starts building a Codec for a custom type.

You need to pass a pattern matching function, built like this:

type Semaphore
    = Red Int String
    | Yellow Float
    | Green


semaphoreCodec : Codec Semaphore
semaphoreCodec =
    Codec.custom
        (\red yellow green value ->
            case value of
                Red i s ->
                    red i s


                Yellow f ->
                    yellow f


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

Define a variant with 0 parameters for a custom type. The first argument is

Define a variant with 1 parameters for a custom type.

Define a variant with 2 parameters for a custom type.

Define a variant with 3 parameters for a custom type.

Define a variant with 4 parameters for a custom type.

Define a variant with 5 parameters for a custom type.

Define a variant with 6 parameters for a custom type.

Define a variant with 7 parameters for a custom type.

Define a variant with 8 parameters for a custom type.

Build a Codec for a fully specified custom type.

Try a set of decoders (in order). The first argument is used for encoding and decoding, the list of other codecs is used as a fallback while decoding.

This is particularly useful for backwards compatibility. You would pass the current codec as the first argument, and the old ones (eventually mapped) as a fallback list to use while decoding.

Transform a Codec.

Create a Codec that produces null as JSON and always decodes as the same value.

Create a Codec for a recursive data structure. The argument to the function you need to pass is the fully formed Codec.

Ignore the JSON and make the decoder fail. This is handy when used with oneOf or andThen where you want to give a custom error message in some case. The encoder will produce null.

Create codecs that depend on previous results.

This is useful for recursive structures that are not easily modeled with recursive. Have a look at the Json.Decode docs for examples.

Create a Codec that doesn't transform the JSON value, just brings it to and from Elm as a Value.

Build your own custom Codec. Useful if you have pre-existing Decoders you need to use.

Create a Codec that produces null as JSON and always decodes as the same value. Obsolete alias of succeed, will be removed in a future version.