async-graphql/docs/en/src/custom_scalars.md

# Custom scalars

In `Async-GraphQL` most common scalar types are built in, but you can also create your own scalar types.

Using `async-graphql::Scalar`, you can add support for a scalar when you implement it. You only need to implement parsing and output functions.

The following example defines a 64-bit integer scalar where its input and output are strings. (Note: `Async-graphQL` already supports 64-bit integers and uses strings as input and output.)

```rust
use async_graphql::*;


struct StringNumber(i64);

#[Scalar]
impl ScalarType for StringNumber {
    fn type_name() -> &'static str {
        // Name of type
        "StringNumber"
    }

    fn parse(value: &Value) -> Option<Self> {
        if let Value::String(value) = value {
            // Parse the integer value
            value.parse().ok().map(StringNumber)
        } else {
            // If the type does not match, return None
            None
        }
    }

    fn to_json(&self) -> Result<serde_json::Value> {
        Ok(serde_json::to_value(self.0).unwrap())
    }
}

```