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<h1 id="scalars"><a class="header" href="#scalars">Scalars</a></h1>
<p><a href="https://spec.graphql.org/October2021#sec-Scalars">GraphQL scalars</a> represent primitive leaf values in a GraphQL type system: numbers, strings, and booleans.</p>
<h2 id="built-in"><a class="header" href="#built-in">Built-in</a></h2>
<p><a href="https://docs.rs/juniper">Juniper</a> provides support for all the <a href="https://spec.graphql.org/October2021/#sec-Scalars.Built-in-Scalars">built-in scalars</a>.</p>
<div class="table-wrapper"><table><thead><tr><th><a href="https://www.rust-lang.org">Rust</a> types</th><th><a href="https://graphql.org">GraphQL</a> scalar</th></tr></thead><tbody>
<tr><td><code>bool</code></td><td><code>Boolean</code></td></tr>
<tr><td><code>i32</code></td><td><code>Int</code></td></tr>
<tr><td><code>f64</code></td><td><code>Float</code></td></tr>
<tr><td><code>String</code>, <code>&str</code></td><td><code>String</code></td></tr>
<tr><td><code>juniper::ID</code></td><td><a href="https://spec.graphql.org/October2021#sec-ID"><code>ID</code></a></td></tr>
</tbody></table>
</div>
<blockquote>
<p><strong>NOTE</strong>: <a href="https://spec.graphql.org/October2021#sec-ID"><code>ID</code></a> scalar is <a href="https://spec.graphql.org/October2021#sec-ID">defined in the GraphQL spec</a> as a type that is serialized as a string, but can be parsed from both a string and an integer.</p>
</blockquote>
<blockquote>
<p><strong>TIP</strong>: There is no built-in support for <code>i64</code>, <code>u64</code>, or other <a href="https://www.rust-lang.org">Rust</a> integer types, as the <a href="https://spec.graphql.org/October2021#sel-FAHXJDCAACKB1qb">GraphQL spec doesn't define any built-in scalars for them</a> by default. Instead, to be supported, they should be defined as <a href="#custom">custom scalars</a> in a <a href="https://graphql.org/learn/schema">GraphQL schema</a>.</p>
</blockquote>
<h2 id="custom"><a class="header" href="#custom">Custom</a></h2>
<p>We can create <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom scalars</a> for other primitive values, but they are still <a href="https://spec.graphql.org/October2021#sel-FAHXJDCAACKB1qb">limited in the data types for representation</a>, and only introduce additional semantic meaning. This, also, often requires coordination with the client library, intended to consume the API we're building.</p>
<p><a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">Custom scalars</a> can be defined in <a href="https://docs.rs/juniper">Juniper</a> by using either <a href="https://docs.rs/juniper/0.16.1/juniper/derive.GraphQLScalar.html"><code>#[derive(GraphQLScalar)]</code></a> or <a href="https://docs.rs/juniper/0.16.1/juniper/attr.graphql_scalar.html"><code>#[graphql_scalar]</code></a> attributes, which do work pretty much the same way (except, <a href="https://docs.rs/juniper/0.16.1/juniper/derive.GraphQLScalar.html"><code>#[derive(GraphQLScalar)]</code></a> cannot be used on <a href="https://doc.rust-lang.org/reference/items/type-aliases.html">type aliases</a>).</p>
<h3 id="transparent-delegation"><a class="header" href="#transparent-delegation">Transparent delegation</a></h3>
<p>Quite often, we want to create a <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom GraphQL scalar</a> type by just wrapping an existing one, inheriting all its behavior. In <a href="https://www.rust-lang.org">Rust</a>, this is often called as <a href="https://rust-unofficial.github.io/patterns/patterns/behavioural/newtype.html">"newtype pattern"</a>. This may be achieved by providing a <code>#[graphql(transparent)]</code> attribute to the definition:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{graphql_scalar, GraphQLScalar};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(transparent)]
pub struct UserId(i32);
// Using `#[graphql_scalar]` attribute here makes no difference, and is fully
// interchangeable with `#[derive(GraphQLScalar)]`. It's only up to the
// personal preference - which one to use.
#[graphql_scalar]
#[graphql(transparent)]
pub struct MessageId {
value: i32,
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<p>That's it, now the <code>UserId</code> and <code>MessageId</code> <a href="https://spec.graphql.org/October2021#sec-Scalars">scalars</a> can be used in <a href="https://graphql.org/learn/schema">GraphQL schema</a>.</p>
<p>We may also customize the definition, to provide more information about our <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom scalar</a> in <a href="https://graphql.org/learn/schema">GraphQL schema</a>:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::GraphQLScalar;
</span><span class="boring">
</span>/// You can use a Rust doc comment to specify a description in GraphQL schema.
#[derive(GraphQLScalar)]
#[graphql(
transparent,
// Overwrite the name of this type in the GraphQL schema.
name = "MyUserId",
// Specifying a type description via attribute takes precedence over the
// Rust doc comment, which allows to separate Rust API docs from GraphQL
// schema descriptions, if required.
description = "Actual description.",
// Optional specification URL.
specified_by_url = "https://tools.ietf.org/html/rfc4122",
)]
pub struct UserId(String);
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<h3 id="resolving"><a class="header" href="#resolving">Resolving</a></h3>
<p>In case we need to customize <a href="https://spec.graphql.org/October2021#sec-Value-Resolution">resolving</a> of a <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom GraphQL scalar</a> value (change the way it gets executed), the <code>#[graphql(to_output_with = <fn path>)]</code> attribute is the way to do so:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{GraphQLScalar, ScalarValue, Value};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(to_output_with = to_output, transparent)]
struct Incremented(i32);
/// Increments [`Incremented`] before converting into a [`Value`].
fn to_output<S: ScalarValue>(v: &Incremented) -> Value<S> {
let inc = v.0 + 1;
Value::from(inc)
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<h3 id="input-value-parsing"><a class="header" href="#input-value-parsing">Input value parsing</a></h3>
<p>Customization of a <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom GraphQL scalar</a> value parsing is possible via <code>#[graphql(from_input_with = <fn path>)]</code> attribute:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{GraphQLScalar, InputValue, ScalarValue};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(from_input_with = Self::from_input, transparent)]
struct UserId(String);
impl UserId {
/// Checks whether the [`InputValue`] is a [`String`] beginning with `id: `
/// and strips it.
fn from_input<S>(input: &InputValue<S>) -> Result<Self, String>
where
S: ScalarValue
{
input.as_string_value()
.ok_or_else(|| format!("Expected `String`, found: {input}"))
.and_then(|str| {
str.strip_prefix("id: ")
.ok_or_else(|| {
format!(
"Expected `UserId` to begin with `id: `, \
found: {input}",
)
})
})
.map(|id| Self(id.to_owned()))
}
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<h3 id="token-parsing"><a class="header" href="#token-parsing">Token parsing</a></h3>
<p>Customization of which tokens a <a href="https://spec.graphql.org/October2021#sec-Scalars">custom GraphQL scalar</a> type should be parsed from, is possible via <code>#[graphql(parse_token_with = <fn path>)]</code> or <code>#[graphql(parse_token(<types>)]</code> attributes:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{
</span><span class="boring"> GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue,
</span><span class="boring"> ScalarValue, ScalarToken, Value,
</span><span class="boring">};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(
to_output_with = to_output,
from_input_with = from_input,
parse_token_with = parse_token,
)]
// ^^^^^^^^^^^^^^^^ Can be replaced with `parse_token(String, i32)`, which
// tries to parse as `String` first, and then as `i32` if
// prior fails.
enum StringOrInt {
String(String),
Int(i32),
}
fn to_output<S: ScalarValue>(v: &StringOrInt) -> Value<S> {
match v {
StringOrInt::String(s) => Value::scalar(s.to_owned()),
StringOrInt::Int(i) => Value::scalar(*i),
}
}
fn from_input<S: ScalarValue>(v: &InputValue<S>) -> Result<StringOrInt, String> {
v.as_string_value()
.map(|s| StringOrInt::String(s.into()))
.or_else(|| v.as_int_value().map(StringOrInt::Int))
.ok_or_else(|| format!("Expected `String` or `Int`, found: {v}"))
}
fn parse_token<S: ScalarValue>(value: ScalarToken<'_>) -> ParseScalarResult<S> {
<String as ParseScalarValue<S>>::from_str(value)
.or_else(|_| <i32 as ParseScalarValue<S>>::from_str(value))
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<blockquote>
<p><strong>NOTE</strong>: Once we provide all 3 custom functions, there is no sense to follow <a href="https://rust-unofficial.github.io/patterns/patterns/behavioural/newtype.html">newtype pattern</a> anymore, as nothing left to inherit.</p>
</blockquote>
<h3 id="full-behavior"><a class="header" href="#full-behavior">Full behavior</a></h3>
<p>Instead of providing all custom functions separately, it's possible to provide a module holding the appropriate <code>to_output()</code>, <code>from_input()</code> and <code>parse_token()</code> functions via <code>#[graphql(with = <module path>)]</code> attribute:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{
</span><span class="boring"> GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue,
</span><span class="boring"> ScalarValue, ScalarToken, Value,
</span><span class="boring">};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(with = string_or_int)]
enum StringOrInt {
String(String),
Int(i32),
}
mod string_or_int {
use super::*;
pub(super) fn to_output<S: ScalarValue>(v: &StringOrInt) -> Value<S> {
match v {
StringOrInt::String(s) => Value::scalar(s.to_owned()),
StringOrInt::Int(i) => Value::scalar(*i),
}
}
pub(super) fn from_input<S: ScalarValue>(v: &InputValue<S>) -> Result<StringOrInt, String> {
v.as_string_value()
.map(|s| StringOrInt::String(s.into()))
.or_else(|| v.as_int_value().map(StringOrInt::Int))
.ok_or_else(|| format!("Expected `String` or `Int`, found: {v}"))
}
pub(super) fn parse_token<S: ScalarValue>(t: ScalarToken<'_>) -> ParseScalarResult<S> {
<String as ParseScalarValue<S>>::from_str(t)
.or_else(|_| <i32 as ParseScalarValue<S>>::from_str(t))
}
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<p>A regular <code>impl</code> block is also suitable for that:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{
</span><span class="boring"> GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue,
</span><span class="boring"> ScalarValue, ScalarToken, Value,
</span><span class="boring">};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
// #[graphql(with = Self)] <- default behaviour, so can be omitted
enum StringOrInt {
String(String),
Int(i32),
}
impl StringOrInt {
fn to_output<S: ScalarValue>(&self) -> Value<S> {
match self {
Self::String(s) => Value::scalar(s.to_owned()),
Self::Int(i) => Value::scalar(*i),
}
}
fn from_input<S>(v: &InputValue<S>) -> Result<Self, String>
where
S: ScalarValue
{
v.as_string_value()
.map(|s| Self::String(s.into()))
.or_else(|| v.as_int_value().map(Self::Int))
.ok_or_else(|| format!("Expected `String` or `Int`, found: {v}"))
}
fn parse_token<S>(value: ScalarToken<'_>) -> ParseScalarResult<S>
where
S: ScalarValue
{
<String as ParseScalarValue<S>>::from_str(value)
.or_else(|_| <i32 as ParseScalarValue<S>>::from_str(value))
}
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<p>At the same time, any custom function still may be specified separately, if required:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">use juniper::{
</span><span class="boring"> GraphQLScalar, InputValue, ParseScalarResult, ScalarValue,
</span><span class="boring"> ScalarToken, Value
</span><span class="boring">};
</span><span class="boring">
</span>#[derive(GraphQLScalar)]
#[graphql(
with = string_or_int,
parse_token(String, i32)
)]
enum StringOrInt {
String(String),
Int(i32),
}
mod string_or_int {
use super::*;
pub(super) fn to_output<S>(v: &StringOrInt) -> Value<S>
where
S: ScalarValue,
{
match v {
StringOrInt::String(s) => Value::scalar(s.to_owned()),
StringOrInt::Int(i) => Value::scalar(*i),
}
}
pub(super) fn from_input<S>(v: &InputValue<S>) -> Result<StringOrInt, String>
where
S: ScalarValue,
{
v.as_string_value()
.map(|s| StringOrInt::String(s.into()))
.or_else(|| v.as_int_value().map(StringOrInt::Int))
.ok_or_else(|| format!("Expected `String` or `Int`, found: {v}"))
}
// No need in `parse_token()` function.
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<blockquote>
<p><strong>TIP</strong>: See more available features in the API docs of the <a href="https://docs.rs/juniper/0.16.1/juniper/derive.GraphQLScalar.html"><code>#[derive(GraphQLScalar)]</code></a> and <a href="https://docs.rs/juniper/0.16.1/juniper/attr.graphql_scalar.html"><code>#[graphql_scalar]</code></a> attributes.</p>
</blockquote>
<h2 id="foreign"><a class="header" href="#foreign">Foreign</a></h2>
<p>For implementing <a href="https://spec.graphql.org/October2021#sec-Scalars.Custom-Scalars">custom scalars</a> on foreign types there is <a href="https://docs.rs/juniper/0.16.1/juniper/attr.graphql_scalar.html"><code>#[graphql_scalar]</code></a> attribute.</p>
<blockquote>
<p><strong>NOTE</strong>: To satisfy <a href="https://doc.rust-lang.org/reference/items/implementations.html#orphan-rules">orphan rules</a>, we should provide a local <a href="https://docs.rs/juniper/0.16.1/juniper/trait.ScalarValue.html"><code>ScalarValue</code></a> implementation.</p>
</blockquote>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">extern crate juniper;
</span><span class="boring">mod date {
</span><span class="boring"> pub struct Date;
</span><span class="boring"> impl std::str::FromStr for Date {
</span><span class="boring"> type Err = String;
</span><span class="boring">
</span><span class="boring"> fn from_str(_value: &str) -> Result<Self, Self::Err> {
</span><span class="boring"> unimplemented!()
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> impl std::fmt::Display for Date {
</span><span class="boring"> fn fmt(&self, _f: &mut std::fmt::Formatter) -> std::fmt::Result {
</span><span class="boring"> unimplemented!()
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">use juniper::DefaultScalarValue as CustomScalarValue;
</span>use juniper::{graphql_scalar, InputValue, ScalarValue, Value};
#[graphql_scalar(
with = date_scalar,
parse_token(String),
scalar = CustomScalarValue,
)]
// ^^^^^^^^^^^^^^^^^ local `ScalarValue` implementation
type Date = date::Date;
// ^^^^^^^^^^ type from another crate
mod date_scalar {
use super::*;
pub(super) fn to_output(v: &Date) -> Value<CustomScalarValue> {
Value::scalar(v.to_string())
}
pub(super) fn from_input(v: &InputValue<CustomScalarValue>) -> Result<Date, String> {
v.as_string_value()
.ok_or_else(|| format!("Expected `String`, found: {v}"))
.and_then(|s| s.parse().map_err(|e| format!("Failed to parse `Date`: {e}")))
}
}
<span class="boring">
</span><span class="boring">fn main() {}</span></code></pre></pre>
<h3 id="supported-out-of-the-box"><a class="header" href="#supported-out-of-the-box">Supported out-of-the-box</a></h3>
<p><a href="https://docs.rs/juniper">Juniper</a> provides out-of-the-box <a href="https://spec.graphql.org/October2021#sec-Scalars">GraphQL scalar</a> implementations for some very common <a href="https://www.rust-lang.org">Rust</a> crates. The types from these crates will be usable in your schemas automatically after enabling the correspondent self-titled <a href="https://doc.rust-lang.org/cargo/reference/features.html">Cargo feature</a>.</p>
<div class="table-wrapper"><table><thead><tr><th><a href="https://www.rust-lang.org">Rust</a> type</th><th><a href="https://graphql.org">GraphQL</a> scalar</th><th><a href="https://doc.rust-lang.org/cargo/reference/features.html">Cargo feature</a></th></tr></thead><tbody>
<tr><td><a href="https://docs.rs/bigdecimal"><code>BigDecimal</code></a></td><td><code>BigDecimal</code></td><td><a href="https://docs.rs/bigdecimal"><code>bigdecimal</code></a></td></tr>
<tr><td><a href="https://docs.rs/bson/latest/bson/oid/struct.ObjectId.html"><code>bson::oid::ObjectId</code></a></td><td><code>ObjectId</code></td><td><a href="https://docs.rs/bson"><code>bson</code></a></td></tr>
<tr><td><a href="https://docs.rs/bson/latest/bson/struct.DateTime.html"><code>bson::DateTime</code></a></td><td><code>UtcDateTime</code></td><td><a href="https://docs.rs/bson"><code>bson</code></a></td></tr>
<tr><td><a href="https://docs.rs/chrono/latest/chrono/naive/struct.NaiveDate.html"><code>chrono::NaiveDate</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/date"><code>Date</code></a></td><td><a href="https://docs.rs/chrono"><code>chrono</code></a></td></tr>
<tr><td><a href="https://docs.rs/chrono/latest/chrono/naive/struct.NaiveTime.html"><code>chrono::NaiveTime</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/local-time"><code>LocalTime</code></a></td><td><a href="https://docs.rs/chrono"><code>chrono</code></a></td></tr>
<tr><td><a href="https://docs.rs/chrono/latest/chrono/naive/struct.NaiveDateTime.html"><code>chrono::NaiveDateTime</code></a></td><td><code>LocalDateTime</code></td><td><a href="https://docs.rs/chrono"><code>chrono</code></a></td></tr>
<tr><td><a href="https://docs.rs/chrono/latest/chrono/struct.DateTime.html"><code>chrono::DateTime</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/date-time"><code>DateTime</code></a></td><td><a href="https://docs.rs/chrono"><code>chrono</code></a></td></tr>
<tr><td><a href="https://docs.rs/chrono-tz/latest/chrono_tz/enum.Tz.html"><code>chrono_tz::Tz</code></a></td><td><code>TimeZone</code></td><td><a href="https://docs.rs/chrono-tz"><code>chrono-tz</code></a></td></tr>
<tr><td><a href="https://docs.rs/rust_decimal/latest/rust_decimal/struct.Decimal.html"><code>Decimal</code></a></td><td><code>Decimal</code></td><td><a href="https://docs.rs/rust_decimal"><code>rust_decimal</code></a></td></tr>
<tr><td><a href="https://docs.rs/time/latest/time/struct.Date.html"><code>time::Date</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/date"><code>Date</code></a></td><td><a href="https://docs.rs/time"><code>time</code></a></td></tr>
<tr><td><a href="https://docs.rs/time/latest/time/struct.Time.html"><code>time::Time</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/local-time"><code>LocalTime</code></a></td><td><a href="https://docs.rs/time"><code>time</code></a></td></tr>
<tr><td><a href="https://docs.rs/time/latest/time/struct.PrimitiveDateTime.html"><code>time::PrimitiveDateTime</code></a></td><td><code>LocalDateTime</code></td><td><a href="https://docs.rs/time"><code>time</code></a></td></tr>
<tr><td><a href="https://docs.rs/time/latest/time/struct.OffsetDateTime.html"><code>time::OffsetDateTime</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/date-time"><code>DateTime</code></a></td><td><a href="https://docs.rs/time"><code>time</code></a></td></tr>
<tr><td><a href="https://docs.rs/time/latest/time/struct.UtcOffset.html"><code>time::UtcOffset</code></a></td><td><a href="https://graphql-scalars.dev/docs/scalars/utc-offset"><code>UtcOffset</code></a></td><td><a href="https://docs.rs/time"><code>time</code></a></td></tr>
<tr><td><a href="https://docs.rs/url"><code>Url</code></a></td><td><code>Url</code></td><td><a href="https://docs.rs/url"><code>url</code></a></td></tr>
<tr><td><a href="https://docs.rs/uuid"><code>Uuid</code></a></td><td><code>Uuid</code></td><td><a href="https://docs.rs/uuid"><code>uuid</code></a></td></tr>
</tbody></table>
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