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<h1 id="implicit-and-explicit-null"><a class="header" href="#implicit-and-explicit-null">Implicit and explicit <code>null</code></a></h1>
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<blockquote>
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<p><a href="https://graphql.org">GraphQL</a> has two semantically different ways to represent the lack of a value:</p>
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<ul>
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<li>Explicitly providing the literal value: <strong>null</strong>.</li>
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<li>Implicitly not providing a value at all.</li>
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</ul>
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</blockquote>
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<p>There are two ways that a client can submit a <a href="https://spec.graphql.org/October2021#sec-Null-Value"><code>null</code> value</a> as an <a href="https://spec.graphql.org/October2021#sec-Language.Arguments">argument</a> or a <a href="https://spec.graphql.org/October2021#sec-Language.Fields">field</a> in a <a href="https://graphql.org">GraphQL</a> query:</p>
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<ol>
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<li>Either use an explicit <code>null</code> literal:
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<pre><code class="language-graphql">{
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field(arg: null)
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}
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</code></pre>
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</li>
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<li>Or simply omit the <a href="https://spec.graphql.org/October2021#sec-Language.Arguments">argument</a>, so the implicit default <code>null</code> value kicks in:
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<pre><code class="language-graphql">{
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field
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}
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</code></pre>
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</li>
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</ol>
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<p>There are some situations where it's useful to know which one exactly has been provided.</p>
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<p>For example, let's say we have a function that allows users to perform a "patch" operation on themselves. Let's say our users can optionally have favorite and least favorite numbers, and the input for that might look like this:</p>
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<pre><pre class="playground"><code class="language-rust edition2021">/// Updates user attributes. Fields that are [`None`] are left as-is.
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struct UserPatch {
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/// If [`Some`], updates the user's favorite number.
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favorite_number: Option<Option<i32>>,
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/// If [`Some`], updates the user's least favorite number.
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least_favorite_number: Option<Option<i32>>,
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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>To set a user's favorite number to 7, we would set <code>favorite_number</code> to <code>Some(Some(7))</code>. In <a href="https://graphql.org">GraphQL</a>, that might look like this:</p>
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<pre><code class="language-graphql">mutation { patchUser(patch: { favoriteNumber: 7 }) }
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</code></pre>
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<p>To unset the user's favorite number, we would set <code>favorite_number</code> to <code>Some(None)</code>. In <a href="https://graphql.org">GraphQL</a>, that might look like this:</p>
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<pre><code class="language-graphql">mutation { patchUser(patch: { favoriteNumber: null }) }
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</code></pre>
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<p>And if we want to leave the user's favorite number alone, just set it to <code>None</code>. In <a href="https://graphql.org">GraphQL</a>, that might look like this:</p>
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<pre><code class="language-graphql">mutation { patchUser(patch: {}) }
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</code></pre>
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<p>The last two cases rely on being able to distinguish between <a href="https://spec.graphql.org/October2021#sel-EAFdRDHAAEJDAoBxzT">explicit and implicit <code>null</code></a>.</p>
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<p>Unfortunately, plain <code>Option</code> is not capable to distinguish them. That's why in <a href="https://docs.rs/juniper">Juniper</a>, this can be done using the <a href="https://docs.rs/juniper/latest/juniper/enum.Nullable.html"><code>Nullable</code></a> type:</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>use juniper::{graphql_object, FieldResult, GraphQLInputObject, Nullable};
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#[derive(GraphQLInputObject)]
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struct UserPatchInput {
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favorite_number: Nullable<i32>,
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least_favorite_number: Nullable<i32>,
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}
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impl From<UserPatchInput> for UserPatch {
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fn from(input: UserPatchInput) -> Self {
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Self {
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// The `explicit()` function transforms the `Nullable` into an
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// `Option<Option<T>>` as expected by the business logic layer.
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favorite_number: input.favorite_number.explicit(),
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least_favorite_number: input.least_favorite_number.explicit(),
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}
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}
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}
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<span class="boring">struct UserPatch {
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</span><span class="boring"> favorite_number: Option<Option<i32>>,
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</span><span class="boring"> least_favorite_number: Option<Option<i32>>,
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">struct Session;
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</span><span class="boring">impl Session {
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</span><span class="boring"> fn patch_user(&self, _patch: UserPatch) -> FieldResult<()> { Ok(()) }
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</span><span class="boring">}
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</span><span class="boring">
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</span>struct Context {
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session: Session,
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}
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impl juniper::Context for Context {}
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struct Mutation;
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#[graphql_object]
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#[graphql(context = Context)]
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impl Mutation {
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fn patch_user(patch: UserPatchInput, ctx: &Context) -> FieldResult<bool> {
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ctx.session.patch_user(patch.into())?;
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Ok(true)
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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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