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|
========================
GraphQL Haskell Tutorial
========================
.. contents::
:depth: 3
..
Getting started
===============
Welcome to graphql-haskell!
We have written a small tutorial to help you (and ourselves) understand
the graphql package.
Since this file is a literate haskell file, we start by importing some
dependencies.
.. code:: haskell
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE LambdaCase #-}
module Main where
import Prelude hiding (empty, putStrLn)
import Data.GraphQL
import Data.GraphQL.Schema
import qualified Data.GraphQL.Schema as Schema
import Control.Applicative
import Data.List.NonEmpty (NonEmpty((:|)))
import Data.Text hiding (empty)
import Data.Aeson
import Data.ByteString.Lazy.Char8 (putStrLn)
import Data.Time
import Debug.Trace
First example
-------------
Now, as our first example, we are going to look at the example from
`graphql.js <https://github.com/graphql/graphql-js>`__.
First we build a GraphQL schema.
.. code:: haskell
schema1 :: Alternative f => Schema f
schema1 = hello :| []
hello :: Alternative f => Resolver f
hello = Schema.scalar "hello" ("it's me" :: Text)
This defines a simple schema with one type and one field, that resolves
to a fixed value.
Next we define our query.
.. code:: haskell
query1 :: Text
query1 = "{ hello }"
To run the query, we call the ``graphql`` with the schema and the query.
.. code:: haskell
main1 :: IO ()
main1 = putStrLn =<< encode <$> graphql schema1 query1
This runs the query by fetching the one field defined, returning
``{"data" : {"hello":"it's me"}}``
Monadic actions
---------------
For this example, we're going to be using time.
.. code:: haskell
schema2 :: Schema IO
schema2 = time :| []
time :: Resolver IO
time = Schema.scalarA "time" $ \case
[] -> do t <- getCurrentTime
return $ show t
_ -> empty
This defines a simple schema with one type and one field, which resolves
to the current time.
Next we define our query.
.. code:: haskell
query2 :: Text
query2 = "{ time }"
main2 :: IO ()
main2 = putStrLn =<< encode <$> graphql schema2 query2
This runs the query, returning the current time
``{"data": {"time":"2016-03-08 23:28:14.546899 UTC"}}``
Errors
------
Errors are handled according to the spec, with fields that cause erros
being resolved to ``null``, and an error being added to the error list.
An example of this is the following query:
.. code:: haskell
queryShouldFail :: Text
queryShouldFail = "{ boyhowdy }"
Since there is no ``boyhowdy`` field in our schema, it will not resolve,
and the query will fail, as we can see in the following example.
.. code:: haskell
mainShouldFail :: IO ()
mainShouldFail = do
r <- graphql schema1 query1
putStrLn $ encode r
putStrLn "This will fail"
r <- graphql schema1 queryShouldFail
putStrLn $ encode r
This outputs:
::
{"data": {"hello": "it's me"}}
This will fail
{"data": {"boyhowdy": null}, "errors":[{"message": "the field boyhowdy did not resolve."}]}
Combining resolvers
-------------------
Now that we have two resolvers, we can define a schema which uses them
both.
.. code:: haskell
schema3 :: Schema IO
schema3 = hello :| [time]
query3 :: Text
query3 = "query timeAndHello { time hello }"
main3 :: IO ()
main3 = putStrLn =<< encode <$> graphql schema3 query3
This queries for both time and hello, returning
``{ "data": {"hello":"it's me","time":"2016-03-08 23:29:11.62108 UTC"}}``
Notice that we can name our queries, as we did with ``timeAndHello``.
Since we have only been using single queries, we can use the shorthand
``{ time hello}``, as we have been doing in the previous examples.
In GraphQL there can only be one operation per query.
Further examples
================
More examples on queries and a more complex schema can be found in the
test directory, in the `Test.StarWars <../../tests/Test/StarWars>`__
module. This includes a more complex schema, and more complex queries.
|
