graphql_ppx

Reason/OCaml PPX (PreProcessor eXtension) helping with creating type-safe, compile time validated GraphQL queries generating response decoders.

This project builds upon mhallin/graphql_ppx. It wouldn't be possible without great work of mhallin/graphql_ppx contributors.

Installation

First, add it to you dependencies using npm or yarn:

yarn add @baransu/graphql_ppx_re --dev
# or
npm install @baransu/graphql_ppx_re  --saveDev

Second, add it to ppx-flags in your bsconfig.json:

"ppx-flags": ["@baransu/graphql_ppx_re/ppx"]

Native

If you want to use native version edit your esy.json file

{
  "dependencies": {
    "graphql_ppx": "*"
  },
  "resolutions": {
    "graphql_ppx": "reasonml-community/graphql_ppx:esy.json#<use latest stable commit from master>"
  }
}

and update your dune file:

(preprocess (pps graphql_ppx))

Usage

This plugin requires a graphql_schema.json file to exist somewhere in the project hierarchy, containing the result of sending an introspection query to your backend. The easiest way to do this is by using get-graphql-schema:

npx get-graphql-schema ENDPOINT_URL -j > graphql_schema.json

Ignore .graphql_ppx_cache in your version control

graphql_ppx will generate a .graphql_ppx_cache folder alongside your JSON schema to optimize parsing performance. If you're using a version control system, you don't need to check it in.

Limitations

While graphql_ppx covers a large portion of the GraphQL spec, there are still some unsupported areas:

• Not all GraphQL validations are implemented. It will not validate argument types and do other sanity-checking of the queries. The fact that a query compiles does not mean that it will pass server-side validation.
• Fragment support is limited and not 100% safe - because graphql_ppx only can perform local reasoning on queries, you can construct queries with fragments that are invalid.

Features

• Objects are converted into Js.t objects
• Enums are converted into polymorphic variants
• Floats, ints, strings, booleans, id are converted into their corresponding native Reason/OCaml types.
• Custom scalars are parsed as Js.Json.t
• Arguments with input objects
• Using @skip and @include will force non-optional fields to become optional.
• Unions are converted to polymorphic variants, with exhaustiveness checking. This only works for object types, not for unions containing interfaces.
• Interfaces are also converted into polymorphic variants. Overlapping interface selections and other more uncommon use cases are not yet supported.
• Basic fragment support
• Required arguments validation - you're not going to miss required arguments on any field.

Extra features

By using some directives prefixed bs, graphql_ppx lets you modify how the result of a query is parsed. All these directives will be removed from the query at compile time, so your server doesn't have to support them.

Record conversion

While Js.t objects often have their advantages, they also come with some limitations. For example, you can't create new objects using the spread (...) syntax or pattern match on their contents. Since they are not named, they also result in quite large type error messages when there are mismatches.

Reason/OCaml records, on the other hand, can be pattern matched, created using the spread syntax, and give nicer error messages when they mismatch. graphql_ppx gives you the option to decode a field as a record using the @bsRecord directive:

type hero = {
  name: string,
  height: number,
  mass: number
};

module HeroQuery = [%graphql7 {|
{
  hero @bsRecord {
    name
    height
    mass
  }
}
|}];

Note that the record has to already exist and be in scope for this to work. graphql_ppx will not create the record. Even though this involves some duplication of both names and types, type errors will be generated if there are any mismatches.

Custom field decoders

If you've got a custom scalar, or just want to convert e.g. an integer to a string to properly fit a record type (see above), you can use the @bsDecoder directive to insert a custom function in the decoder:

module HeroQuery = [%graphql7 {|
{
  hero {
    name
    height @bsDecoder(fn: "string_of_float")
    mass
  }
}
|}];

In this example, height will be converted from a float to a string in the result. Using the fn argument, you can specify any function literal you want.

Non-union variant conversion

If you've got an object which in practice behaves like a variant - like signUp above, where you either get a user or a list of errors - you can add a @bsVariant directive to the field to turn it into a polymorphic variant:

module SignUpQuery = [%graphql7
  {|
mutation($name: String!, $email: String!, $password: String!) {
  signUp(email: $email, email: $email, password: $password) @bsVariant {
    user {
      name
    }

    errors {
      field
      message
    }
  }
}
|}
];

let _ =
  SignUpQuery.make(
    ~name="My name",
    ~email="email@example.com",
    ~password="secret",
    (),
  )
  |> Api.sendQuery
  |> Promise.then_(response =>
       (
         switch (response##signUp) {
         | `User(user) => Js.log2("Signed up a user with name ", user##name)
         | `Errors(errors) => Js.log2("Errors when signing up: ", errors)
         }
       )
       |> Promise.resolve
     );

This helps with the fairly common pattern for mutations that can fail with user-readable errors.

Alternative Query.make syntax

When you define a query with variables, the make function will take corresponding labelled arguments. This is convenient when constructing and sending the queries yourself, but might be problematic when trying to abstract over multiple queries.

For this reason, another function called makeWithVariables is also generated. This function takes a single Js.t object containing all variables.

module MyQuery = [%graphql7
  {|
  mutation ($username: String!, $password: String!) {
    ...
  }
|}
];

/* You can either use `make` with labelled arguments: */
let query = MyQuery.make(~username="testUser", password = "supersecret", ());

/* Or, you can use `makeWithVariables`: */
let query =
  MyQuery.makeWithVariables({
    "username": "testUser",
    "password": "supersecret",
  });

Getting the type of the parsed value

If you want to get the type of the parsed and decoded value - useful in places where you can't use Reason/OCaml's type inference - use the t type of the query module:

module MyQuery = [%graphql7 {| { hero { name height }} |}];

/* This is something like Js.t({ . hero: Js.t({ name: string, weight: float }) }) */
type resultType = MyQuery.t;

Troubleshooting

"Type ... doesn't have any fields"

Sometimes when working with union types you'll get the following error.

Fatal error: exception Graphql_ppx_base__Schema.Invalid_type("Type IssueTimelineItems doesn't have any fields")

This is an example of a query that will result in such error:

nodes {
  __typename
  ... on ClosedEvent {
    closer {
      __typename
      ... on PullRequest {
        id
        milestone { id }
      }
    }
  }
}

This is because we allow querying union fields only in certain cases. GraphQL provides the __typename field but it's not present in GraphQL introspection query thus graphql_ppx doesn't know that this field exists. To fix your query simply remove __typename. It's added behinds a scene as an implementation detail and serves us as a way to decide which case to select when parsing your query result.

This is an example of a correct query:

nodes {
  ... on ClosedEvent {
    closer {
      ... on PullRequest {
        id
        milestone { id }
      }
    }
  }
}

Configuration

If you need to customize certain features of graphql_ppx you can provide ppx arguments to do so:

-apollo-mode

By default graphql_ppx adds __typename only to fields on which we need those informations (Unions and Interfaces). If you want to add __typename on every object in a query you can specify it by using -apollo-mode in ppx-flags. It's usefull in case of using apollo-client because of it's cache.

"ppx-flags": [
  ["@baransu/graphql_ppx_re/ppx", "-apollo-mode",]
],

-schema

By default graphql_ppx uses graphql_schema.json file from your root directory. You can override it by providing -schema argument in ppx-flags to overriding it.

"ppx-flags": [
  ["@baransu/graphql_ppx_re/ppx", "-schema ../graphql_schema.json"]
],

Query specific configuration

If you want to use multiple schemas in your project it can be provided as a secondary config argument in your graphql ppx definition.

module MyQuery = [%graphql7
  {|
    query pokemon($id: String, $name: String) {
      pokemon(name: $name, id: $id) {
        id
        name
      }
    }
  |};
  {schema: "pokedex_schema.json"}
];

This will use the pokedex_schema.json instead of using the default graphql_schema.json file.

This opens up the possibility to use multiple different GraphQL APIs in the same project.

Note the path to your file is based on where you run bsb. In this case pokedex_schema.json is a sibling to node_modules.

Supported platforms

graphql_ppx somes with prebuild binaries for linux-x64, darwin-x64 and win-x64. If you need support for other platform, please open an issue.

Contributing

Developing

npm install -g esy@latest
esy install
esy build

Running tests

BuckleScript

cd tests_bucklescript
npm test

Native

For native run:

esy dune runtest -f