Decode required fields.

Example:

user : Decoder User
user =
    require "id" Decode.int <| \id ->
    require "name" Decode.string <| \name ->


    Decode.succeed
        { id = id
        , name = name
        }

In this example the decoder will fail if:

• The JSON value is not an object.
• Any of the fields "id" or "name" are missing. If the object contains other fields they are ignored and will not cause the decoder to fail.
• The value of field "id" is not an Int.
• The value of field "name" is not a String.

Decode nested fields. Works the same as require but on nested fieds.

blogPost : Decoder BlogPost
blogPost =
    require "id" Decode.int <| \id ->
    require "title" Decode.string <| \title ->
    requireAt ["author", "name"] Decode.string <| \authorName ->


    Decode.succeed
        { id = id
        , title = title
        , author = authorName
        }

Decode optional fields.

Always decodes to a Maybe value and never fails.

Example:

person : Decoder Person
person =
    require "name" Decode.string <| \name ->
    attempt "weight" Decode.int <| \maybeWeight ->


    Decode.succeed
        { name = name
        , weight = maybeWeight
        }

In this example the maybeWeight value will be Nothing if:

• The JSON value was not an object
• The weight field is missing.
• The weight field is not an Int.

In this case there is no difference between a field being null or missing. If a field must exist but can be null, use require and Decode.maybe instead:

require "field" (Decode.maybe Decode.string) <| \field ->

Decode optional nested fields. Works the same way as attempt but on nested fields.