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In Sway and the FuelVM, Bytes32 represents hashes. They hold a 256-bit (32-byte) value. Bytes32 is a wrapper on a 32-sized slice of u8: pub struct Bytes32([u8; 32]);.

These are the main ways of creating a Bytes32:

use std::str::FromStr;
use fuels::types::Bytes32;
// Zeroed Bytes32
let b256 = Bytes32::zeroed();
// Grab the inner `[u8; 32]` from
// `Bytes32` by dereferencing (i.e. `*`) it.
assert_eq!([0u8; 32], *b256);
// From a `[u8; 32]`.
let my_slice = [1u8; 32];
let b256 = Bytes32::new(my_slice);
assert_eq!([1u8; 32], *b256);
// From a hex string.
let hex_str = "0x0000000000000000000000000000000000000000000000000000000000000000";
let b256 = Bytes32::from_str(hex_str)?;
assert_eq!([0u8; 32], *b256);

Bytes32 also implements the fmt module's Debug, Display, LowerHex and UpperHex traits. For example, you can get the display and hex representations with:

let b256_string = b256.to_string();
let b256_hex_string = format!("{b256:#x}");

For a full list of implemented methods and traits, see the fuel-types documentation Icon Link.

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Note: In Fuel, there's a special type called b256, which is similar to Bytes32; also used to represent hashes, and it holds a 256-bit value. In Rust, through the SDK, this is represented as Bits256(value) where value is a [u8; 32]. If your contract method takes a b256 as input, all you need to do is pass a Bits256([u8; 32]) when calling it from the SDK.