Eugen Wissner
d12577ae71
Returning resolvers from other resolvers isn't supported anymore. Since we have a type system now, we define the resolvers in the object type fields and pass an object with the previous result to them.
169 lines
4.4 KiB
Plaintext
169 lines
4.4 KiB
Plaintext
---
|
|
title: GraphQL Haskell Tutorial
|
|
---
|
|
|
|
|
|
== 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.
|
|
|
|
> {-# LANGUAGE OverloadedStrings #-}
|
|
> module Main where
|
|
>
|
|
> import Control.Monad.IO.Class (liftIO)
|
|
> import Data.Aeson (encode)
|
|
> import Data.ByteString.Lazy.Char8 (putStrLn)
|
|
> import qualified Data.HashMap.Strict as HashMap
|
|
> import Data.Text (Text)
|
|
> import qualified Data.Text as Text
|
|
> import Data.Time (getCurrentTime)
|
|
>
|
|
> import Language.GraphQL
|
|
> import Language.GraphQL.Trans
|
|
> import Language.GraphQL.Type
|
|
> import qualified Language.GraphQL.Type.Out as Out
|
|
>
|
|
> import Prelude hiding (putStrLn)
|
|
|
|
=== 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.
|
|
|
|
> schema1 :: Schema IO
|
|
> schema1 = Schema queryType Nothing
|
|
>
|
|
> queryType :: ObjectType IO
|
|
> queryType = ObjectType "Query" Nothing []
|
|
> $ HashMap.singleton "hello"
|
|
> $ Field Nothing (Out.NamedScalarType string) mempty hello
|
|
>
|
|
> hello :: ActionT IO Value
|
|
> hello = pure $ String "it's me"
|
|
|
|
This defines a simple schema with one type and one field, that resolves to a fixed value.
|
|
|
|
Next we define our query.
|
|
|
|
> query1 :: Text
|
|
> query1 = "{ hello }"
|
|
|
|
|
|
To run the query, we call the `graphql` with the schema and the query.
|
|
|
|
> 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.
|
|
|
|
> schema2 :: Schema IO
|
|
> schema2 = Schema queryType2 Nothing
|
|
>
|
|
> queryType2 :: ObjectType IO
|
|
> queryType2 = ObjectType "Query" Nothing []
|
|
> $ HashMap.singleton "time"
|
|
> $ Field Nothing (Out.NamedScalarType string) mempty time
|
|
>
|
|
> time :: ActionT IO Value
|
|
> time = do
|
|
> t <- liftIO getCurrentTime
|
|
> pure $ String $ Text.pack $ show t
|
|
|
|
This defines a simple schema with one type and one field,
|
|
which resolves to the current time.
|
|
|
|
Next we define our query.
|
|
|
|
> 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:
|
|
|
|
> 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.
|
|
|
|
> mainShouldFail :: IO ()
|
|
> mainShouldFail = do
|
|
> success <- graphql schema1 query1
|
|
> putStrLn $ encode success
|
|
> putStrLn "This will fail"
|
|
> failure <- graphql schema1 queryShouldFail
|
|
> putStrLn $ encode failure
|
|
>
|
|
|
|
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.
|
|
|
|
> schema3 :: Schema IO
|
|
> schema3 = Schema queryType3 Nothing
|
|
>
|
|
> queryType3 :: ObjectType IO
|
|
> queryType3 = ObjectType "Query" Nothing [] $ HashMap.fromList
|
|
> [ ("hello", Field Nothing (Out.NamedScalarType string) mempty hello)
|
|
> , ("time", Field Nothing (Out.NamedScalarType string) mempty 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.
|
|
|
|
> main :: IO ()
|
|
> main = main1 >> main2 >> mainShouldFail >> main3
|