package graphql_parser

GraphQL Servers in OCaml

This repo contains a library for creating GraphQL servers in OCaml. Note that the API is still under active development.

Current feature set:

• Type-safe schema design
• GraphQL parser in pure OCaml using angstrom (April 2016 RFC draft)
• Query execution
• Introspection of schemas
• Arguments for fields
• Allows variables in queries
• Lwt support
• Async support
• Example with HTTP server and GraphiQL

Documentation

Three OPAM packages are provided:

• graphql provides the core functionality and is IO-agnostic. It provides a functor Graphql.Schema.Make(IO) to instantiate with your own IO monad.
• graphql-lwt provides the module Graphql_lwt.Schema with Lwt support in field resolvers.
• graphql-async provides the module Graphql_async.Schema with Async support in field resolvers.

API documentation:

• graphql
• graphql-lwt
• graphql-async

Examples

GraphiQL

To run a sample GraphQL server also serving GraphiQL, do the following:

opam install graphql-lwt jbuilder
git checkout git@github.com/andreas/ocaml-graphql-server.git
cd ocaml-graphql-server
cd examples
jbuilder build server.exe
./_build/default/server.exe

Now open http://localhost:8080.

Defining a Schema

open Graphql

type role = User | Admin
type user = {
  id   : int;
  name : string;
  role : role;
}

let users = [
  { id = 1; name = "Alice"; role = Admin };
  { id = 2; name = "Bob"; role = User }
]

let role = Schema.(enum "role"
  ~doc:"The role of a user"
  ~values:[
    enum_value "USER" ~value:User;
    enum_value "ADMIN" ~value:Admin;
  ]
)

let user = Schema.(obj "user"
  ~doc:"A user in the system"
  ~fields:(fun _ -> [
    field "id"
      ~doc:"Unique user identifier"
      ~typ:(non_null int)
      ~args:Arg.[]
      ~resolve:(fun () p -> p.id)
    ;
    field "name"
      ~typ:(non_null string)
      ~args:Arg.[]
      ~resolve:(fun () p -> p.name)
    ;
    field "role"
      ~typ:(non_null role)
      ~args:Arg.[]
      ~resolve:(fun () p -> p.role)
  ])
)

let schema = Schema.(schema [
  field "users"
    ~typ:(non_null (list (non_null user)))
    ~args:Arg.[]
    ~resolve:(fun () () -> users)
])

Running a Query

Without variables:

let query = Graphql_parser.parse "{ users { name } }" in
Graphql.Schema.execute schema ctx query

With variables parsed from JSON:

let query = Graphql_parser.parse "{ users(limit: $x) { name } }" in
let json_variables = Yojson.Basic.(from_string "{\"x\": 42}" |> Util.to_assoc) in
let variables = (json_variables :> (string * Graphql_parser.const_value) list)
Graphql.Schema.execute schema ctx ~variables query

Self-Recursive Objects

To allow defining an object that refers to itself, the type itself is provided as argument to the ~fields function. Example:

type tweet = {
  id : int;
  replies : tweet list;
}

let tweet = Schema.(obj "tweet"
  ~fields:(fun tweet -> [
    field "id"
      ~typ:(non_null int)
      ~args:Arg.[]
      ~resolver:(fun ctx t -> t.id)
    ;
    field "replies"
      ~typ:(non_null (list tweet))
      ~args:Arg.[]
      ~resolver:(fun ctx t -> t.replies)
  ])
)

Mutually Recursive Objects

Mutually recursive objects can be defined using let rec and lazy:

let rec foo = lazy Schema.(obj "foo"
  ~fields:(fun _ -> [
    field "bar"
      ~typ:Lazy.(force bar)
      ~args.Arg.[]
      ~resolver:(fun ctx foo -> foo.bar)
  ])
and bar = lazy Schema.(obj "bar"
  ~fields:(fun _ -> [
    field "foo"
      ~typ:Lazy.(force foo)
      ~args.Arg.[]
      ~resolver:(fun ctx bar -> bar.foo)
  ])

Lwt Support

open Lwt.Infix
open Graphql_lwt

let schema = Schema.(schema [
  io_field "wait"
    ~typ:(non_null float)
    ~args:Arg.[
      arg "duration" ~typ:float;
    ]
    ~resolve:(fun () () -> Lwt_unix.sleep duration >|= fun () -> duration)
])

Async Support

open Core.Std
open Async.Std
open Graphql_async

let schema = Schema.(schema [
  io_field "wait"
    ~typ:(non_null float)
    ~args:Arg.[
      arg "duration" ~typ:float;
    ]
    ~resolve:(fun () () -> after (Time.Span.of_float duration) >>| fun () -> duration)
])

Arguments

Arguments for a field can either be required, optional or optional with a default value:

Schema.(obj "math"
  ~fields:(fun _ -> [
    field "sum"
      ~typ:int
      ~args:Arg.[
        arg  "x" ~typ:(non_null int); (* <-- required *)
        arg  "y" ~typ:int;            (* <-- optional *)
        arg' "z" ~typ:int ~default:7  (* <-- optional w/ default *)
      ]
      ~resolve:(fun () () x y z ->
        let y' = match y with Some n -> n | None -> 42 in
        x + y' + z
      )
  ])
)

Note that you must use arg' to provide a default value.

Design

Only valid schemas should pass the type checker. If a schema compiles, the following holds:

1. The type of a field agrees with the return type of the resolve function.
2. The arguments of a field agrees with the accepted arguments of the resolve function.
3. The source of a field agrees with the type of the object to which it belongs.
4. The context argument for all resolver functions in a schema agree.

The following types ensure this:

type ('ctx, 'src) obj = {
  name   : string;
  fields : ('ctx, 'src) field list Lazy.t;
}
and ('ctx, 'src) field =
  Field : {
    name    : string;
    typ     : ('ctx, 'out) typ;
    args    : ('out, 'args) Arg.arg_list;
    resolve : 'ctx -> 'src -> 'args;
  } -> ('ctx, 'src) field
and ('ctx, 'src) typ =
  | Object      : ('ctx, 'src) obj -> ('ctx, 'src option) typ
  | List        : ('ctx, 'src) typ -> ('ctx, 'src list option) typ
  | NonNullable : ('ctx, 'src option) typ -> ('ctx, 'src) typ
  | Scalar      : 'src scalar -> ('ctx, 'src option) typ
  | Enum        : 'src enum -> ('ctx, 'src option) typ

The type parameters can be interpreted as follows:

• 'ctx is a value that is passed all resolvers when executing a query against a schema,
• 'src is the domain-specific source value, e.g. a user record,
• 'args is the arguments of the resolver, and will be of the type 'arg¹ -> ... -> 'argⁿ -> 'out,
• 'out is the result of the resolver, which must agree with the type of the field.

Particularly noteworthy is ('ctx, 'src) field, which hides the type 'out. The type 'out is used to ensure that the output of a resolver function agrees with the input type of the field's type.

For introspection, three additional types are used to hide the type parameters 'ctx and src:

  type any_typ =
    | AnyTyp : (_, _) typ -> any_typ
    | AnyArgTyp : (_, _) Arg.arg_typ -> any_typ
  type any_field =
    | AnyField : (_, _) field -> any_field
    | AnyArgField : (_, _) Arg.arg -> any_field
  type any_arg = AnyArg : (_, _) Arg.arg -> any_ar

This is to avoid "type parameter would avoid it's scope"-errors.