Generates Elm types, JSON decoders, JSON encoders, and Fuzz tests from JSON schema specifications.
Only supports - a subset of - JSON Schema draft v7.
This project requires that you already have elixir
and its build tool mix installed, this can be done with brew install elixir
or similar.
- Download latest release at: https://github.com/dragonwasrobot/json-schema-to-elm/releases, or
- clone this repository:
git clone git@github.com:dragonwasrobot/json-schema-to-elm.git, then - build an executable:
MIX_ENV=prod mix build(Windowscmd.exe:set "MIX_ENV=prod" && mix build), and - run the executable,
./js2e(Windows:escript .\js2e), that has now been created in your current working directory.
Run ./js2e for usage instructions.
Note: The
js2etool only tries to resolve references for the file(s) you pass it. So if you need to generate Elm code from more than one file you have to pass it the enclosing directory of the relevant JSON schema files, in order for it to be able to resolve the references correctly.
A proper description of which properties are mandatory and how the generator
works is still in progress, but feel free to take a look at the examples
folder which contains an example of a pair of JSON schemas and their
corresponding Elm output. Likewise, representations of each of the different
JSON schema types are described in the lib/types folder.
If we supply js2e with the following JSON schema file, definitions.json:
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "Definitions",
"$id": "http://example.com/definitions.json",
"description": "Schema for common types",
"definitions": {
"color": {
"$id": "#color",
"type": "string",
"enum": [ "red", "yellow", "green", "blue" ]
},
"point": {
"$id": "#point",
"type": "object",
"properties": {
"x": {
"type": "number"
},
"y": {
"type": "number"
}
},
"required": [ "x", "y" ]
}
}
}it produces the following Elm file, Data/Definitions.elm:
module Data.Definitions exposing
( Color(..)
, Point
, colorDecoder
, encodeColor
, encodePoint
, pointDecoder
)
-- Schema for common types
import Helper.Encode as Encode
import Json.Decode as Decode exposing (Decoder)
import Json.Decode.Extra as Decode
import Json.Decode.Pipeline
exposing
( custom
, optional
, required
)
import Json.Encode as Encode exposing (Value)
type Color
= Red
| Yellow
| Green
| Blue
type alias Point =
{ x : Float
, y : Float
}
colorDecoder : Decoder Color
colorDecoder =
Decode.string |> Decode.andThen (parseColor >> Decode.fromResult)
parseColor : String -> Result String Color
parseColor color =
case color of
"red" ->
Ok Red
"yellow" ->
Ok Yellow
"green" ->
Ok Green
"blue" ->
Ok Blue
_ ->
Err <| "Unknown color type: " ++ color
pointDecoder : Decoder Point
pointDecoder =
Decode.succeed Point
|> required "x" Decode.float
|> required "y" Decode.float
encodeColor : Color -> Value
encodeColor color =
color |> colorToString |> Encode.string
colorToString : Color -> String
colorToString color =
case color of
Red ->
"red"
Yellow ->
"yellow"
Green ->
"green"
Blue ->
"blue"
encodePoint : Point -> Value
encodePoint point =
[]
|> Encode.required "x" point.x Encode.float
|> Encode.required "y" point.y Encode.float
|> Encode.objectwhich contains an Elm type for the color and point definitions along with
their corresponding JSON decoders and encoders.
Furthermore, if we instead supply js2e with a directory of JSON schema files
that have references across files, e.g.
{
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "http://example.com/circle.json",
"title": "Circle",
"description": "Schema for a circle shape",
"type": "object",
"properties": {
"center": {
"$ref": "http://example.com/definitions.json#point"
},
"radius": {
"type": "number"
},
"color": {
"$ref": "http://example.com/definitions.json#color"
}
},
"required": ["center", "radius"]
}then the corresponding Elm file, Data/Circle.elm, will import the
definitions (types, encoders and decoders) from the other Elm module,
Data/Definitions.elm.
module Data.Circle exposing
( Circle
, circleDecoder
, encodeCircle
)
-- Schema for a circle shape
import Data.Definitions as Definitions
import Helper.Encode as Encode
import Json.Decode as Decode exposing (Decoder)
import Json.Decode.Extra as Decode
import Json.Decode.Pipeline
exposing
( custom
, optional
, required
)
import Json.Encode as Encode exposing (Value)
type alias Circle =
{ center : Definitions.Point
, color : Maybe Definitions.Color
, radius : Float
}
circleDecoder : Decoder Circle
circleDecoder =
succeed Circle
|> required "center" Definitions.pointDecoder
|> optional "color" (Decode.nullable Definitions.colorDecoder) Nothing
|> required "radius" Decode.float
encodeCircle : Circle -> Value
encodeCircle circle =
[]
|> Encode.required "center" circle.center Definitions.encodePoint
|> Encode.optional "color" circle.color Definitions.encodeColor
|> Encode.required "radius" circle.radius Encode.float
|> Encode.objectFurthermore, js2e also generates test files for the generated decoders and
encoders to make the generated code immediately testable. The generated test
files fuzzes instances of a given Elm type and tests that encoding it as JSON
and decoding it back into Elm returns the original instance of that generated
Elm type. In the above case, the following test files,
tests/Data/CircleTests.elm and tests/Data/DefinitionsTests.elm, are
generated:
module Data.CircleTests exposing
( circleFuzzer
, encodeDecodeCircleTest
)
-- Tests: Schema for a circle shape
import Data.Circle exposing (..)
import Data.DefinitionsTests as Definitions
import Expect exposing (Expectation)
import Fuzz exposing (Fuzzer)
import Json.Decode as Decode
import Test exposing (..)
circleFuzzer : Fuzzer Circle
circleFuzzer =
Fuzz.map3
Circle
Definitions.pointFuzzer
(Fuzz.maybe Definitions.colorFuzzer)
Fuzz.niceFloat
encodeDecodeCircleTest : Test
encodeDecodeCircleTest =
fuzz circleFuzzer "can encode and decode Circle object" <|
\circle ->
circle
|> encodeCircle
|> Decode.decodeValue circleDecoder
|> Expect.equal (Ok circle)and
module Data.DefinitionsTests exposing
( colorFuzzer
, encodeDecodeColorTest
, encodeDecodePointTest
, pointFuzzer
)
-- Tests: Schema for common types
import Data.Definitions exposing (..)
import Expect exposing (Expectation)
import Fuzz exposing (Fuzzer)
import Json.Decode as Decode
import Test exposing (..)
colorFuzzer : Fuzzer Color
colorFuzzer =
[ Red, Yellow, Green, Blue ]
|> List.map Fuzz.constant
|> Fuzz.oneOf
encodeDecodeColorTest : Test
encodeDecodeColorTest =
fuzz colorFuzzer "can encode and decode Color object" <|
\color ->
color
|> encodeColor
|> Decode.decodeValue colorDecoder
|> Expect.equal (Ok color)
pointFuzzer : Fuzzer Point
pointFuzzer =
Fuzz.map2
Point
Fuzz.niceFloat
Fuzz.niceFloat
encodeDecodePointTest : Test
encodeDecodePointTest =
fuzz pointFuzzer "can encode and decode Point object" <|
\point ->
point
|> encodePoint
|> Decode.decodeValue pointDecoder
|> Expect.equal (Ok point)Finally, js2e also generates package config files, package.json and
elm.json, and a .tool-versions file, making it easy to test that the
generated Elm code is behaving as expected. Note that the .tool-versions file
is not a file required by elm nor elm-test but instead a file used by the
asdf version manager, https://github.com/asdf-vm/asdf, to install and run the
correct compiler versions of node and elm specified in the .tool-versions
file for a given project.
Thus, if we supply the following directory structure to js2e in the above
case:
.
└── js2e_input/
├── definitions.json
└── circle.json
the following new directory structure is generated:
.
└── js2e_output/
├── .tool-versions
├── package.json
├── elm.json
├── src/
│ ├── Data/
│ │ ├── Circle.elm
│ │ └── Definitions.elm
│ └── Helper/
│ └── Encode.elm
└── tests/
└── Data/
├── CircleTests.elm
└── DefinitionsTests.elm
containing the files described above along with the needed package config files to compile and run the tests.
Any errors encountered by the js2e tool while parsing the JSON schema files or
printing the Elm code output, is reported in an Elm-like style, e.g.
--- UNKNOWN NODE TYPE -------------------------------------- all_of_example.json
The value of "type" at '#/allOf/0/properties/description' did not match a known node type
"type": "strink"
^^^^^^^^
Was expecting one of the following types
["null", "boolean", "object", "array", "number", "integer", "string"]
Hint: See the specification section 6.25. "Validation keywords - type"
<http://json-schema.org/latest/json-schema-validation.html#rfc.section.6.25>
or
--- UNRESOLVED REFERENCE ----------------------------------- all_of_example.json
The following reference at `#/allOf/0/color` could not be resolved
"$ref": #/definitions/kolor
^^^^^^^^^^^^^^^^^^^
Hint: See the specification section 9. "Base URI and dereferencing"
<http://json-schema.org/latest/json-schema-core.html#rfc.section.9>
If you encounter an error while using js2e that does not mimic the above
Elm-like style, but instead looks like an Elixir stacktrace, please report this
as a bug by opening an issue and including a JSON schema example that recreates
the error.
If you feel like something is missing/wrong or if I've misinterpreted the JSON schema spec, feel free to open an issue so we can discuss a solution. Note that the JSON schema parser has been moved to the new project, https://github.com/dragonwasrobot/json_schema, so this repo only implements the Elm code generators.
Please consult CONTRIBUTING.md first before opening an issue.