From c2d2ee6b85991026f77db6deba1fedcfa44217b5 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Sun, 5 Mar 2023 08:25:15 +0100
Subject: [PATCH] =?UTF-8?q?=E2=80=9ETutorial=E2=80=9C=20hinzuf=C3=BCgen?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 Tutorial.md | 148 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 Tutorial.md

diff --git a/Tutorial.md b/Tutorial.md
new file mode 100644
index 0000000..3b4047e
--- /dev/null
+++ b/Tutorial.md
@@ -0,0 +1,148 @@
+# GraphQL Haskell Tutorial
+
+## Getting started
+
+Welcome to GraphQL!
+
+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.
+
+```haskell
+{-# 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.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.
+
+```haskell
+schema1 :: Schema IO
+schema1 = schema queryType Nothing Nothing mempty
+
+queryType :: ObjectType IO
+queryType = ObjectType "Query" Nothing []
+    $ HashMap.singleton "hello"
+    $ ValueResolver helloField hello
+
+helloField :: Field IO
+helloField = Field Nothing (Out.NamedScalarType string) mempty
+
+hello :: Resolve IO
+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.
+
+```haskell
+query1 :: Text
+query1 = "{ hello }"
+```
+
+To run the query, we call the `graphql` with the schema and the query.
+
+```haskell
+main1 :: IO ()
+main1 = graphql schema1 query1
+    >>= either (const $ pure ()) (putStrLn . encode)
+```
+
+This runs the query by fetching the one field defined, returning
+
+```json
+{"data" : {"hello":"it's me"}}
+```
+
+
+### Monadic actions
+
+For this example, we're going to be using time.
+
+```haskell
+schema2 :: Schema IO
+schema2 = schema queryType2 Nothing Nothing mempty
+
+queryType2 :: ObjectType IO
+queryType2 = ObjectType "Query" Nothing []
+    $ HashMap.singleton "time"
+    $ ValueResolver timeField time
+
+timeField :: Field IO
+timeField = Field Nothing (Out.NamedScalarType string) mempty
+
+time :: Resolve IO
+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 = graphql schema2 query2
+    >>= either (const $ pure ()) (putStrLn . encode)
+```
+
+This runs the query, returning the current time
+
+```json
+{"data": {"time":"2016-03-08 23:28:14.546899 UTC"}}
+```
+
+
+### Combining resolvers
+
+Now that we have two resolvers, we can define a schema which uses them both.
+
+```haskell
+schema3 :: Schema IO
+schema3 = schema queryType3 Nothing Nothing mempty
+
+queryType3 :: ObjectType IO
+queryType3 = ObjectType "Query" Nothing [] $ HashMap.fromList
+    [ ("hello", ValueResolver helloField hello)
+    , ("time", ValueResolver timeField time)
+    ]
+
+query3 :: Text
+query3 = "query timeAndHello { time hello }"
+
+main3 :: IO ()
+main3 = graphql schema3 query3
+    >>= either (const $ pure ()) (putStrLn . encode)
+```
+
+This queries for both time and hello, returning
+
+```json
+{ "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.
\ No newline at end of file