package graphql_parser
Install
Dune Dependency
Authors
Maintainers
Sources
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README.md.html
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:
[x] Type-safe schema design
[x] GraphQL parser in pure OCaml using angstrom (April 2016 RFC draft)
[x] Query execution
[x] Introspection of schemas
[x] Arguments for fields
[x] Allows variables in queries
[x] Lwt support
[x] Async support
[x] Example with HTTP server and GraphiQL
[x] GraphQL Subscriptions
Documentation
Four OPAM packages are provided:
graphql
provides the core functionality and is IO-agnostic. It provides a functorGraphql.Schema.Make(IO)
to instantiate with your own IO monad.graphql-lwt
provides the moduleGraphql_lwt.Schema
with Lwt support in field resolvers.graphql-async
provides the moduleGraphql_async.Schema
with Async support in field resolvers.graphql_parser
provides query parsing functionality.graphql-cohttp
allows exposing a schema over HTTP using Cohttp.
API documentation:
Examples
GraphiQL
To run a sample GraphQL server also serving GraphiQL, do the following:
opam install graphql-lwt dune
git clone git@github.com:andreas/ocaml-graphql-server.git
cd ocaml-graphql-server/examples
dune exec ./server.exe
Now open http://localhost:8080/graphql.
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 info p -> p.id)
;
field "name"
~typ:(non_null string)
~args:Arg.[]
~resolve:(fun info p -> p.name)
;
field "role"
~typ:(non_null role)
~args:Arg.[]
~resolve:(fun info p -> p.role)
])
)
let schema = Schema.(schema [
field "users"
~typ:(non_null (list (non_null user)))
~args:Arg.[]
~resolve:(fun info () -> users)
])
Running a Query
Without variables:
match Graphql_parser.parse "{ users { name } }" with
| Ok query -> Graphql.Schema.execute schema ctx query
| Error err -> failwith err
With variables parsed from JSON:
match Graphql_parser.parse "{ users(limit: $x) { name } }" with
| Ok query ->
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
| Error err ->
failwith err
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.[]
~resolve:(fun info t -> t.id)
;
field "replies"
~typ:(non_null (list tweet))
~args:Arg.[]
~resolve:(fun info 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.[]
~resolve:(fun info foo -> foo.bar)
])
and bar = lazy Schema.(obj "bar"
~fields:(fun _ -> [
field "foo"
~typ:Lazy.(force foo)
~args.Arg.[]
~resolve:(fun info 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 info () ->
Lwt_result.ok (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 info () ->
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 info () 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.
Subscriptions
Schema.(schema [
...
]
~subscriptions:[
subscription_field "user_created"
~typ:(non_null user)
~resolve:(fun info ->
let user_stream, push_to_user_stream = Lwt_stream.create () in
let destroy_stream = (fun () -> push_to_user_stream None) in
Lwt_result.return (user_stream, destroy_stream))
])
HTTP Server
Using Lwt:
open Graphql_lwt
let schema = Schema.(schema [
...
])
module Graphql_cohttp_lwt = Graphql_cohttp.Make (Schema) (Cohttp_lwt.Body)
let () =
let callback = Graphql_cohttp_lwt.make_callback (fun _req -> ()) schema in
let server = Cohttp_lwt_unix.Server.make ~callback () in
let mode = `TCP (`Port 8080) in
Cohttp_lwt_unix.Server.create ~mode server
|> Lwt_main.run
Design
Only valid schemas should pass the type checker. If a schema compiles, the following holds:
The type of a field agrees with the return type of the resolve function.
The arguments of a field agrees with the accepted arguments of the resolve function.
The source of a field agrees with the type of the object to which it belongs.
The context argument for all resolver functions in a schema agree.
The following blog posts introduces the core design concepts:
https://andreas.github.io/2017/11/29/type-safe-graphql-with-ocaml-part-1/
https://andreas.github.io/2018/01/05/modeling-graphql-type-modifiers-with-gadts/