A JSON schema for dragonwasrobot/json-schema-to-elm#23

example.elm

module Domain.SomeTitle exposing (..)

-- 

import Json.Decode as Decode
    exposing
        ( succeed
        , fail
        , map
        , maybe
        , field
        , at
        , andThen
        , oneOf
        , nullable
        , Decoder
        )
import Json.Decode.Pipeline
    exposing
        ( decode
        , required
        , optional
        , custom
        )
import Json.Encode as Encode
    exposing
        ( Value
        , object
        )


type alias SomeTitle =
    { first : String
    , second : String
    , third : Maybe Third
    }








type alias Third =
    { a : String
    }


SomeTitleDecoder : Decoder SomeTitle
SomeTitleDecoder =
    decode SomeTitle
        |> required "first" Decode.string
        |> required "second" Decode.string
        |> optional "third" (nullable thirdDecoder) Nothing








thirdDecoder : Decoder Third
thirdDecoder =
    decode Third
        |> required "a" Decode.string


encodeSomeTitle : SomeTitle -> Value
encodeSomeTitle SomeTitle =
    let
        first =
            [ ( "first", Encode.string SomeTitle.first ) ]

        second =
            [ ( "second", Encode.string SomeTitle.second ) ]

        third =
            case SomeTitle.third of
                Just third ->
                    [ ( "third", encodeThird third ) ]

                Nothing ->
                    []
    in
        object <|
            first ++ second ++ third








encodeThird : Third -> Value
encodeThird third =
    let
        a =
            [ ( "a", Encode.string third.a ) ]
    in
        object <|
            a

example.schema.json

{
  "$schema": "http://json-schema.org/draft-04/schema",
  "id": "https://github.com/reitzig/example",
  "title": "Some title",
  "type": "object",
  "required": ["first", "second"],
  "additionalProperties": false,
  
  
  "properties": {
    "first": {
      "title": "First property",
      "type": "string",
      "pattern": "[0-9]{4}-[0-9]{2}-[0-9]{2}"
    },

    "second": {
      "title": "The license of this version of the rules",
      "type": "string"
    },
    
    "third": {
      "title": "The language of the contained texts",
      "type": "object",
      "required": ["a"],
      
      "properties": {
        "a": { 
          "type": "string",
          "pattern": "[a-z]{2}(_[a-z]{2})?"
        }
      }
    }
  },
  
  "patternProperties": {
    "^[0-9]+(\\.[0-9]+)*$": {
      "title": "An item",
      "type": "object",
      "anyOf": [
        { "required": ["title"] },
        { "required": ["text"] }
      ],
      "additionalProperties": false,
      
      "properties": {
        "title": {
          "type": "string"
        },
        "text": {
          "type": "string"
        }
      }
    }
  }
}