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<main>
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<h1 id="generics"><a class="header" href="#generics">Generics</a></h1>
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<p>Yet another point where <a href="https://graphql.org">GraphQL</a> and <a href="https://www.rust-lang.org">Rust</a> differs is in how generics work:</p>
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<ul>
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<li>In <a href="https://www.rust-lang.org">Rust</a>, almost any type could be generic - that is, take type parameters.</li>
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<li>In <a href="https://graphql.org">GraphQL</a>, there are only two generic types: <a href="https://spec.graphql.org/October2021#sec-List">lists</a> and <a href="https://spec.graphql.org/October2021#sec-Non-Null">non-<code>null</code>ables</a>.</li>
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</ul>
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<p>This poses a restriction on what we can expose in <a href="https://graphql.org">GraphQL</a> from <a href="https://www.rust-lang.org">Rust</a>: no generic structs can be exposed - all type parameters must be bound. For example, we cannot expose <code>Result<T, E></code> as a <a href="https://spec.graphql.org/October2021#sec-Types">GraphQL type</a>, but we <em>can</em> expose <code>Result<User, String></code> as a <a href="https://spec.graphql.org/October2021#sec-Types">GraphQL type</a>.</p>
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<p>Let's make a slightly more compact but generic implementation of <a href="error/schema.html#example-non-struct-objects">the last schema error example</a>:</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
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</span><span class="boring">use juniper::{graphql_object, GraphQLObject};
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</span><span class="boring">
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</span>#[derive(GraphQLObject)]
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struct User {
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name: String,
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}
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#[derive(GraphQLObject)]
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struct ForumPost {
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title: String,
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}
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#[derive(GraphQLObject)]
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struct ValidationError {
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field: String,
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message: String,
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}
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struct MutationResult<T>(Result<T, Vec<ValidationError>>);
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#[graphql_object]
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#[graphql(name = "UserResult")]
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impl MutationResult<User> {
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fn user(&self) -> Option<&User> {
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self.0.as_ref().ok()
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}
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fn error(&self) -> Option<&[ValidationError]> {
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self.0.as_ref().err().map(Vec::as_slice)
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}
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}
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#[graphql_object]
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#[graphql(name = "ForumPostResult")]
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impl MutationResult<ForumPost> {
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fn forum_post(&self) -> Option<&ForumPost> {
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self.0.as_ref().ok()
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}
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fn error(&self) -> Option<&[ValidationError]> {
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self.0.as_ref().err().map(Vec::as_slice)
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}
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}
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<span class="boring">
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</span><span class="boring">fn main() {}</span></code></pre></pre>
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<p>Here, we've made a wrapper around a <code>Result</code> and exposed some concrete instantiations of <code>Result<T, E></code> as distinct <a href="https://spec.graphql.org/October2021#sec-Objects">GraphQL objects</a>.</p>
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<blockquote>
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<p><strong>NOTE</strong>: The reason we needed the wrapper is of <a href="https://www.rust-lang.org">Rust</a>'s <a href="https://doc.rust-lang.org/reference/items/implementations.html#trait-implementation-coherence">orphan rules</a> (both the <code>Result</code> and <a href="https://docs.rs/juniper">Juniper</a>'s internal traits are from third-party sources).</p>
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</blockquote>
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<blockquote>
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<p><strong>NOTE</strong>: Because we're using generics, we also need to specify a <code>name</code> for our instantiated <a href="https://spec.graphql.org/October2021#sec-Types">GraphQL types</a>. Even if <a href="https://docs.rs/juniper">Juniper</a> <em>could</em> figure out the name, <code>MutationResult<User></code> wouldn't be a <a href="https://spec.graphql.org/October2021#sec-Names">valid GraphQL type name</a>. And, also, two different <a href="https://spec.graphql.org/October2021#sec-Types">GraphQL types</a> cannot have the same <code>MutationResult</code> name, inferred by default.</p>
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