Libraries in Sway are files used to define new common behavior. An example of this is the Sway Core Library which outlines various methods that the u64 type implements.

Writing Libraries

Libraries are denoted using the library keyword at the beginning of the file, followed by a name so that they can be imported, e.g. library foo;.

library my_library;

A good reference library to use when learning library design is the Sway Core Library. The add function interface is defined via the Add trait and then implemented for u64. This attaches this add function to the type so that, when the trait is imported, u64s can utilize the add function.

pub trait Add {
    fn add(self, other: Self) -> Self;

impl Add for u64 {
    fn add(self, other: Self) -> Self {
        asm(r1: self, r2: other, r3) {
            add r3 r2 r1;
            r3: u64

This snippet defines the trait Add, then implements it for the u64 type by providing a function body. This gives all u64s the add function, which is inserted at compile time when you use the + operator in Sway. Libraries can export more than functions, though. You can also use libraries to just export types like below.

pub struct MyStruct {
    field_one: u64,
    field_two: bool,

Libraries are composed of just a Forc.toml file and a src directory, unlike contracts which usually contain a tests directory and a Cargo.toml file as well. An example of a library's Forc.toml:

authors = ["Fuel Labs <>"]
entry = "lib.sw"
license = "Apache-2.0"
name = "lib-std"


which denotes the authors, an entry file, the name by which it can be imported, and any dependencies. For large libraries, it is recommended to have a lib.sw entry point re-export all other sub-libraries. For example, the lib.sw of the standard library looks like:

library std;

dep block;
dep storage;
dep constants;

with other libraries contained in the src folder, like the block library (inside of block.sw):

library block;

/// Get the current block height
pub fn height() -> u64 {
    asm(height) {
        bhei height;
        height: u64

The dep keyword in the main library includes a dependency on another library, making all of its items (such as functions and structs) accessible from the main library. The dep keyword simply makes the library a dependency and fully accessible within the current context.

Using Libraries

Libraries can be imported using the use keyword and with a :: separating the name of the library and the import.

Here is an example of importing storage and its related functions from the standard library.

use std::storage::*;

Wildcard imports using * are supported, but it is always recommended to use explicit imports where possible.

You will also need to link the library in the Forc.toml of the forc repo that you're calling from. You can do this by opening up the Forc.toml file and adding the following line to the bottom:

wallet_lib = { path = "../wallet_lib" }