Perl6-GraphQL

Perl 6 implementation of GraphQL

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Users example

The Schema describes the interface for your application in detail.

You start by describing your Schema in terms of data types. Starting with the GraphQL core types (String, Int, Float, Boolean, ID), possibly modified ([List], Non-Null!), and built into a set of Object Types. You can also define types that are Unions of other types or Enum enumerations of pre-defined values.

For example:

type User {
  id: ID!
  name: String
  birthday: String
  status: Boolean
}

I’ve used ‘String’ as the type for birthday. There isn’t a core GraphQL ‘Date’ or ‘DateTime’ type – though other languages frequently implement it. We’ll probably add that to the Perl6 version soon. For this example, it is just a string.

Descriptions

Though not (yet) part of the standard, another frequently implemented extension is the ability to add descriptions to types and fields with # comments. If you look at the eg/users.schema file, you’ll see # descriptions for some of the types and fields. Those descriptions can be queried with the GraphQL introspection queries through the meta types and queries __Schema, __Type, etc. The GraphiQL IDE displays them while you explore the schema with the Docs functionality.

Types with arguments

Object fields can also include arguments. To query our User database, we’ll define two such queries:

type Query {
  listusers(start: ID = "0", count: Int = "1"): [User]
  user(id: ID!): User
}

So you can list count users starting with a specific id, or just query a single user.

Resolvers

Now that we’ve defined the external GraphQL API, we need to define functions that resolve those queries.

First a Perl class to act like our GraphQL User type:

class User
{
    has Int $.id is rw;
    has Str $.name is rw;
    has Str $.birthday is rw;
    has Bool $.status is rw;
}

and a pseudo-database to hold our users:

my @users = User.new(id => 0, name => '...', birthday => '...', status => True),
            User.new(...),
            ...;

Then a few simple functions to implement listusers() and user(), matching the argument list defined in the GraphQL schema:

my $resolvers = 
{
    Query =>
    {
        listusers => sub (:$start, Int :$count)
        {
            @users[$start ..^ $start+$count]
        },

        user => sub (:$id)
        {
            @users[$id]
        }
    }
};

user() just returns the user specified by $id, and listusers() returns count of them starting with id start and returns them in an Array which gets mapped to the GraphQL List.

Then create your GraphQL::Schema :

my $schema = GraphQL::Schema.new(...schema here..., resolvers => $resolvers);

If you put your schema in a separate file, you can plug it in easily with IO.slurp (see the example).

Running this server under Bailador, you can explore the schema interactively, and execute our queries. For example:

{
  user(id: 0) {
    name
    birthday
  }
}

to see the name and birthday of user 0.

Note that in the hello example, the resolver was specified down to the Field level returning a Scalar (String), while here, the resolvers return an Object or a List of Objects. If you return a Perl 6 Class, it must include methods for resolving each field of the type. (e.g. here, we have methods for name(), birthday(), etc. defined by Perl because they are public). You can actually mix and match if you like, defining individual resolvers for some fields, while relying on Class methods for others. If you want to define both, you’ll have to call $schema.resolvers() multiple times, once with the resolver for the Object level, and once for the fields of that object.

Mutations

Querying is nice, but what if you want to allow changes? (Hopefully only by trusted, authenticated, authorized users.)

In reality, there isn’t really anything special about Mutations, and if you wanted to do an update with a normal query, nothing technical would stop you. It is highly recommended, however, that you group such queries and explicitly declare them as mutations. This has a few real consequences. For one, the server will always execute mutations serially though it is allowed to execute normal queries in parallel. This will prevent race conditions and race induced non-deterministic behaviors. For another, client tools can assume that normal queries can be cached, while mutations never will be.

We can define a special kind of object called an InputObject, that looks almost like a normal type:

input UserInput {
    name: String
    birthday: String
    status: Boolean
}

and define some mutations in the schema:

type Mutation {
  adduser(newuser: UserInput!): ID
  updateuser(id: ID!, userinput: UserInput!): User
}

and implement some matching resolvers for those:

adduser => sub (:%newuser)
{
    push @users, User.new(id => @users.elems, |%newuser);
    return @users.elems - 1;
},

updateuser => sub (:$id, :%userinput)
{
    for %userinput.kv -> $k, $v
    {
        @users[$id]."$k"() = $v;
    }

    return @users[$id]
}

See the full schema in users.schema, and the example Bailador server in usersserver.pl.

Some sample queries

List the first three users, with their names:

{
  listusers(count:3) {
    name
  }
}

Get the birthday and status for user 2:

{
  user(id: 2){
    birthday
    status
  }
}

Add a new user named “John” (null status and birthday because they aren’t specified)

mutation {
  adduser(newuser: {name: "John"})
}

Set John’s birthday to “Every Year”, and return his name and birthday:

mutation {
  updateuser(id: "5", userinput: { birthday: "Every Year" }) {
    name
    birthday
  }
}

Notes

Again, this isn’t really a production quality server. You should do more with authentication/authorization, sessions, setting content-types, etc. This server also ignores variables supplied by the user. Those should also be passed in to the schema execute() call. In the future, some more of that work may be added in to this repository.

Also parsing in Perl 6 is still very slow, so a simple optimization is to cache parsed documents, and just re-use the parsed document if the same query is sent. (When you build GraphQL into User interfaces, the same query documents are frequently reused.)

You can see how that would work:.

Instead of:

$schema.execute('some query');

use the .document() method to parse it, and the :document named parameter to execute to specify an already parsed document (a GraphQL::Document).

my $document = $schema.document('some query');
$schema.execute(:$document);