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<h1 class="title">GraphQL Haskell Tutorial</h1>
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</header>
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<nav id="TOC">
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<ul>
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<li><a href="#getting-started">Getting started</a><ul>
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<li><a href="#first-example">First example</a></li>
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<li><a href="#monadic-actions">Monadic actions</a></li>
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<li><a href="#errors">Errors</a></li>
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<li><a href="#combining-resolvers">Combining resolvers</a></li>
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</ul></li>
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<li><a href="#further-examples">Further examples</a></li>
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</ul>
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</nav>
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<section id="getting-started" class="level2">
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<h2>Getting started</h2>
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<p>Welcome to graphql-haskell!</p>
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<p>We have written a small tutorial to help you (and ourselves) understand the graphql package.</p>
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<p>Since this file is a literate haskell file, we start by importing some dependencies.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">{-# LANGUAGE OverloadedStrings #-}</span>
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<span class="ot">{-# LANGUAGE LambdaCase #-}</span>
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<span class="kw">module</span> <span class="dt">Main</span> <span class="kw">where</span>
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<span class="kw">import </span><span class="dt">Prelude</span> <span class="kw">hiding</span> (empty, putStrLn)
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<span class="kw">import </span><span class="dt">Data.GraphQL</span>
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<span class="kw">import </span><span class="dt">Data.GraphQL.Schema</span>
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<span class="kw">import qualified</span> <span class="dt">Data.GraphQL.Schema</span> <span class="kw">as</span> <span class="dt">Schema</span>
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<span class="kw">import </span><span class="dt">Control.Applicative</span>
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<span class="kw">import </span><span class="dt">Data.Text</span> <span class="kw">hiding</span> (empty)
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<span class="kw">import </span><span class="dt">Data.Aeson</span>
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<span class="kw">import </span><span class="dt">Data.ByteString.Lazy.Char8</span> (putStrLn)
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<span class="kw">import </span><span class="dt">Data.Time</span>
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<span class="kw">import </span><span class="dt">Debug.Trace</span></code></pre></div>
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<section id="first-example" class="level3">
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<h3>First example</h3>
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<p>Now, as our first example, we are going to look at the example from <a href="https://github.com/graphql/graphql-js">graphql.js</a>.</p>
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<p>First we build a GraphQL schema.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">schema1 ::</span> <span class="dt">Alternative</span> f <span class="ot">=></span> <span class="dt">Schema</span> f
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schema1 <span class="fu">=</span> <span class="dt">Schema</span> [hello]
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<span class="ot">hello ::</span> <span class="dt">Alternative</span> f <span class="ot">=></span> <span class="dt">Resolver</span> f
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hello <span class="fu">=</span> Schema.scalar <span class="st">"hello"</span> (<span class="st">"it's me"</span><span class="ot"> ::</span> <span class="dt">Text</span>)</code></pre></div>
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<p>This defines a simple schema with one type and one field, that resolves to a fixed value.</p>
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<p>Next we define our query.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">query1 ::</span> <span class="dt">Text</span>
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query1 <span class="fu">=</span> <span class="st">"{ hello }"</span></code></pre></div>
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<p>To run the query, we call the <code>graphql</code> with the schema and the query.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">main1 ::</span> <span class="dt">IO</span> ()
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main1 <span class="fu">=</span> putStrLn <span class="fu">=<<</span> encode <span class="fu"><$></span> graphql schema1 query1</code></pre></div>
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<p>This runs the query by fetching the one field defined, returning</p>
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<p><code>{"data" : {"hello":"it's me"}}</code></p>
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</section>
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<section id="monadic-actions" class="level3">
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<h3>Monadic actions</h3>
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<p>For this example, we’re going to be using time.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">schema2 ::</span> <span class="dt">Schema</span> <span class="dt">IO</span>
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schema2 <span class="fu">=</span> <span class="dt">Schema</span> [time]
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<span class="ot">time ::</span> <span class="dt">Resolver</span> <span class="dt">IO</span>
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time <span class="fu">=</span> Schema.scalarA <span class="st">"time"</span> <span class="fu">$</span> \<span class="kw">case</span>
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[] <span class="ot">-></span> <span class="kw">do</span> t <span class="ot"><-</span> getCurrentTime
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return <span class="fu">$</span> show t
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_ <span class="ot">-></span> empty</code></pre></div>
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<p>This defines a simple schema with one type and one field, which resolves to the current time.</p>
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<p>Next we define our query.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">query2 ::</span> <span class="dt">Text</span>
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query2 <span class="fu">=</span> <span class="st">"{ time }"</span>
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<span class="ot">main2 ::</span> <span class="dt">IO</span> ()
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main2 <span class="fu">=</span> putStrLn <span class="fu">=<<</span> encode <span class="fu"><$></span> graphql schema2 query2</code></pre></div>
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<p>This runs the query, returning the current time</p>
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<p><code>{"data": {"time":"2016-03-08 23:28:14.546899 UTC"}}</code></p>
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</section>
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<section id="errors" class="level3">
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<h3>Errors</h3>
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<p>Errors are handled according to the spec, with fields that cause erros being resolved to <code>null</code>, and an error being added to the error list.</p>
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<p>An example of this is the following query:</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">queryShouldFail ::</span> <span class="dt">Text</span>
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queryShouldFail <span class="fu">=</span> <span class="st">"{ boyhowdy }"</span></code></pre></div>
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<p>Since there is no <code>boyhowdy</code> field in our schema, it will not resolve, and the query will fail, as we can see in the following example.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">mainShouldFail ::</span> <span class="dt">IO</span> ()
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mainShouldFail <span class="fu">=</span> <span class="kw">do</span>
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r <span class="ot"><-</span> graphql schema1 query1
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putStrLn <span class="fu">$</span> encode r
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putStrLn <span class="st">"This will fail"</span>
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r <span class="ot"><-</span> graphql schema1 queryShouldFail
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putStrLn <span class="fu">$</span> encode r</code></pre></div>
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<p>This outputs:</p>
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<pre><code>{"data": {"hello": "it's me"}}
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This will fail
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{"data": {"boyhowdy": null}, "errors":[{"message": "the field boyhowdy did not resolve."}]}</code></pre>
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</section>
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<section id="combining-resolvers" class="level3">
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<h3>Combining resolvers</h3>
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<p>Now that we have two resolvers, we can define a schema which uses them both.</p>
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<div class="sourceCode"><pre class="sourceCode literate haskell"><code class="sourceCode haskell"><span class="ot">schema3 ::</span> <span class="dt">Schema</span> <span class="dt">IO</span>
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schema3 <span class="fu">=</span> <span class="dt">Schema</span> [hello, time]
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<span class="ot">query3 ::</span> <span class="dt">Text</span>
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query3 <span class="fu">=</span> <span class="st">"query timeAndHello { time hello }"</span>
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<span class="ot">main3 ::</span> <span class="dt">IO</span> ()
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main3 <span class="fu">=</span> putStrLn <span class="fu">=<<</span> encode <span class="fu"><$></span> graphql schema3 query3</code></pre></div>
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<p>This queries for both time and hello, returning</p>
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<p><code>{ "data": {"hello":"it's me","time":"2016-03-08 23:29:11.62108 UTC"}}</code></p>
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<p>Notice that we can name our queries, as we did with <code>timeAndHello</code>. Since we have only been using single queries, we can use the shorthand <code>{ time hello}</code>, as we have been doing in the previous examples.</p>
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<p>In GraphQL there can only be one operation per query.</p>
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</section>
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</section>
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<section id="further-examples" class="level2">
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<h2>Further examples</h2>
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<p>More examples on queries and a more complex schema can be found in the test directory, in the <a href="../../tests/Test/StarWars">Test.StarWars</a> module. This includes a more complex schema, and more complex queries.</p>
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</section>
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