GraphQL implementation in Haskell

This implementation is relatively low-level by design, it doesn't provide any mappings between the GraphQL types and Haskell's type system and avoids compile-time magic. It focuses on flexibility instead, so other solutions can be built on top of it.

State of the work

For now this library provides:

• Parser for the query and schema languages, as well as a printer for the query language (minimizer and pretty-printer).
• Data structures to define a type system.
• Executor (queries, mutations and subscriptions are supported).
• Validation is work in progress.
• Introspection isn't available yet.

But the idea is to be a Haskell port of graphql-js.

For a more precise list of currently missing features see issues.

Documentation

API documentation is available through Hackage.

Further documentation will be made available in the Wiki.

Getting started

We start with a simple GraphQL API that provides us with some famous and less famous cites.

"""
Root Query type.
"""
type Query {
  """
  Provides a cite.
  """
  cite: String!
}

This is called a GraphQL schema, it defines all queries supported by the API. Query is the root query type. Every GraphQL API should define a query type.

Query has a single field cite that returns a String. The ! after the type denotes that the returned value cannot be Null. GraphQL fields are nullable by default.

To be able to work with this schema, we are going to implement it in Haskell.

{-# LANGUAGE OverloadedStrings #-}

import qualified Data.Aeson as Aeson
import qualified Data.ByteString.Lazy.Char8 as ByteString.Lazy.Char8
import qualified Data.HashMap.Strict as HashMap
import Language.GraphQL
import Language.GraphQL.Type
import qualified Language.GraphQL.Type.Out as Out

-- GraphQL supports 3 kinds of operations: queries, mutations and subscriptions.
-- Our first schema supports only queries.
citeSchema :: Schema IO
citeSchema = schema queryType Nothing Nothing mempty

-- GraphQL distinguishes between input and output types. Input types are field
-- argument types and they are defined in Language.GraphQL.Type.In. Output types
-- are result types, they are defined in Language.GraphQL.Type.Out. Root types
-- are always object types.
--
-- Here we define a type "Query". The second argument is an optional
-- description, the third one is the list of interfaces implemented by the
-- object type. The last argument is a field map. Keys are field names, values
-- are field definitions and resolvers. Resolvers are the functions, where the
-- actual logic lives, they return values for the respective fields.
queryType :: Out.ObjectType IO
queryType = Out.ObjectType "Query" (Just "Root Query type.") []
    $ HashMap.singleton "cite" citeResolver
  where
    -- 'ValueResolver' is a 'Resolver' data constructor, it combines a field
    -- definition with its resolver function. This function resolves a value for
    -- a field (as opposed to the 'EventStreamResolver' used by subscriptions).
    -- Our resolver just returns a constant value.
    citeResolver = ValueResolver citeField
        $ pure "Piscis primum a capite foetat"

    -- The first argument is an optional field description. The second one is
    -- the field type and the third one is for arguments (we have none in this
    -- example).
    --
    -- GraphQL has named and wrapping types. String is a scalar, named type.
    -- Named types are nullable by default. To make our "cite" field
    -- non-nullable, we wrap it in the wrapping type, Non-Null.
    citeField = Out.Field
        (Just "Provides a cite.") (Out.NonNullScalarType string) HashMap.empty

-- Now we can execute a query. Since our schema defines only one field,
-- everything we can do is to ask to resolve it and give back the result.
-- Since subscriptions don't return plain values, the 'graphql' function returns
-- an 'Either'. 'Left' is for subscriptions, 'Right' is for queries and
-- mutations.
main :: IO ()
main = do
    Right result <- graphql citeSchema "{ cite }"
    ByteString.Lazy.Char8.putStrLn $ Aeson.encode result

Executing this query produces the following JSON:

{
  "data": {
    "cite": "Piscis primum a capite foetat"
  }
}

Contact

Suggestions, patches and bug reports are welcome.

Should you have questions on usage, please open an issue and ask – this helps to write useful documentation.