Gleam for Rust users

• Comments
• Variables
  • Match operator
  • Variables type annotations
• Functions
  • Exporting functions
  • Function type annotations
  • Function overloading
  • Referencing functions
  • Labelled arguments
• Modules
• Operators
• Constants
• Blocks
• Data types
  • Strings
  • Tuples
  • Lists
• [Patterns] TODO
• Flow control TODO
  • Case TODO
  • Try TODO
• Type aliases TODO
• Custom types
  • Records
  • Unions
  • Opaque custom types TODO
• Modules TODO
  • Imports TODO
  • Nested modules TODO
  • First class modules TODO

Comments

Comments look very similar in both languages.

Rust

In Rust comments are written with a // prefix.

// Hello, Joe!

Comments starting with /// are used to document the following statement. Comments starting with //! are used to document the current module.

//! This module is very important.

/// The answer to life, the universe, and everything.
const answer: u64 = 42;

Gleam

In Gleam comments are written with a // prefix.

// Hello, Joe!

Comments starting with /// are used to document the following statement. Comments starting with //// are used to document the current module.

//// This module is very important.

/// The answer to life, the universe, and everything.
const answer: Int = 42

Variables

You can declare and redeclare variables in both languages by using the let keyword. Variables are immutable in both languages.

Rust

let size = 50;
let size = size + 100;
let size = 1;

Gleam

let size = 50
let size = size + 100
let size = 1

Gleam doesn’t have a mut keyword to mark variables as mutable, they’re always immutable.

Match operator

Rust

In Rust, let and = can be used for pattern matching, but you’ll get compile errors if there’s a type mismatch.

let [x] = [1];
let 2 = x; // compile error
let [y] = "Hello"; // compile error, type mismatch

Gleam

In Gleam, let and = can also be used for pattern matching, but you’ll get compile errors if there’s a type mismatch, and a runtime error if there’s a value mismatch. For assertions, the equivalent let assert keyword is preferred.

let [x] = [1]
let assert 2 = x // runtime error
let assert [y] = "Hello" // compile error, type mismatch

Variables type annotations

Both languages allow you to annotate variables with types in a similar style. The compilers will check that the type matches the variable’s assigned value. Both languages allow you to skip type annotation, and will instead infer the type from the provided value.

Rust

let some_list: [u64; 3] = [1, 2, 3];
let other_list = [1, 2, 3];

Gleam

let some_list: List(Int) = [1, 2, 3]
let other_list = [1, 2, 3]

Functions

Rust

pub fn sum(x: u64, y: u64) -> u64 {
  x + y
}

let mul = |x, y| x * y;
mul(1, 2);

Gleam

Gleam’s functions are declared using a syntax similar to Rust’s. Anonymous functions are a bit different from Rust, using the fn keyword again.

pub fn sum(x, y) {
  x + y
}

let mul = fn(x, y) { x * y }
mul(1, 2)

Exporting functions

Both languages use the same system, where functions are private by default, and need the pub keyword to be marked as public.

Rust

// this is public
pub fn sum(x: u64, y: u64) -> u64 {
    x + y
}

// this is private
fn mul(x: u64, y: u64) -> u64 {
    x * y
}

Gleam

// this is public
pub fn sum(x, y) {
  x + y
}

// this is private
fn mul(x, y) {
  x * y
}

Function type annotations

Rust

Rust functions always need type annotations.

pub fn sum(x: u64, y: u64) -> u64 {
  x + y
}

pub fn mul(x: u64, y: u64) -> u64 {
  x * y
}

Gleam

Functions can optionally have their argument and return types annotated in Gleam. These type annotations will always be checked by the compiler and throw a compilation error if not valid. The compiler will still type check your program using type inference if annotations are omitted.

pub fn add(x: Int, y: Int) -> Int {
  x + y
}

pub fn mul(x: Int, y: Int) -> Bool { // compile error, type mismatch
  x * y
}

Function overloading

Like Rust, Gleam does not support function overloading, so there can only be 1 function with a given name, and the function can only have a single implementation for the types it accepts.

Referencing functions

Referencing functions in Gleam works like in Rust, without any special syntax.

Rust

fn identity(x: u64) -> u64 {
  x
}

fn main() {
  let func = identity;
  func(100);
}

Gleam

fn identity(x) {
  x
}

fn main() {
  let func = identity
  func(100)
}

Labelled arguments

In Gleam arguments can be given a label as well as an internal name. As with Erlang the name used at the call-site does not have to match the name used for the variable inside the function.

There is no performance cost to Gleam’s labelled arguments as they are optimised to regular function calls at compile time, and all the arguments are fully type checked.

pub fn replace(inside string, each pattern, with replacement) {
  go(string, pattern, replacement)
}

replace(each: ",", with: " ", inside: "A,B,C")

There is no equivalent feature in Rust.

Operators

Constants

Rust

In Rust constants can be created using the const keyword, and have to be given a type annotation.

const the_answer: u64 = 42;

pub fn main() {
  the_answer;
}

In Rust, public constants can be referenced from other modules.

mod other_module {
  pub const the_answer: u64 = 42;
}

fn main() {
  other_module::the_answer;
}

Gleam

In Gleam constants can be created using the const keyword, and can be optionally given a type annotation.

const the_answer = 42

pub fn main() {
  the_answer
}

Additionally, public constants can be referenced from other modules.

//// In module other_module.gleam
pub const the_answer: Int = 42

import other_module

fn main() {
  other_module.the_answer
}

Blocks

Rust

In Rust braces { } are used to group expressions, and arithmetic operations are grouped with parenthesis ( ).

let x = {
    println!("{}", 1);
    2
};
let y = x * (x + 10); // parenthesis are used to change arithmetic operations order

Gleam

In Gleam braces { } are used to group both expressions and arithmetic operations.

let x = {
  print(1)
  2
}
let y = x * {x + 10} // braces are used to change arithmetic operations order

Data types

Strings

In both Rust and Gleam all strings are UTF-8 encoded binaries.

Rust

"Hellø, world!"

Gleam

"Hellø, world!"

Tuples

Rust

let my_tuple = ("username", "password", 10);
let (_, password, _) = my_tuple;

Gleam

Tuples are very useful in Gleam as they’re the only collection data type that allows mixed types in the collection.

let my_tuple = #("username", "password", 10)
let #(_, password, _) = my_tuple

Lists

Rust arrays and Gleam lists are similar, but Rust’s are slightly more limited.

Rust

let list = [1, 2, 3];

let other = [0, ..list]; // Compile error!
let [0, second_element, ..] = list; // Compile error!

Gleam

The cons operator works the same way both for pattern matching and for appending elements to the head of a list.

let list = [1, 2, 3]
let list = [0, ..list]
let [0, second_element, ..] = list
[1.0, ..list] // compile error, type mismatch

Custom types

Custom type allows you to define a collection data type with a fixed number of named fields, and the values in those fields can be of differing types.

Rust

Rust has Structs, which are declared using the struct keyword.

struct Person {
  name: String,
  age: u64,
}

let person = Person {
  name: "Jake".to_string(),
  age: 35,
};
let name = person.name;

Gleam

Gleam’s custom types can be declared using the type keyword. At runtime, they have a tuple representation and are compatible with Erlang records.

type Person {
  Person(name: String, age: Int)
}

let person = Person(name: "Jake", age: 35)
let name = person.name

Unions

Rust

Rust’s union type is called Enum and declared with the enum keyword:

enum IpAddress {
  V4(u8, u8, u8, u8),
  V6(String)
}

let addr_v4 = IpAddress::V4(192, 168, 1, 1);
let addr_v6 = IpAddress::V6("::1".to_string());

Gleam

In Gleam, custom types become unions by having multiple constructors:

type IpAddress {
  V4(Int, Int, Int, Int)
  V6(String)
}

let addr_v4 = V4(192, 168, 1, 1)
let addr_v6 = V6("::1")

Flow control

Case

Rust

When you need to match a value against multiple possible patterns, Rust has the match expression. Such matches are e.g. enums or string slices:

enum MyEnum {
  A(i32),
  B,
  C,
}

let my_enum = MyEnum::A(10);

match my_enum {
  MyEnum::A(n) => do_a(n),
  MyEnum::B => do_b(),
  MyEnum::C => {
    do_sth();
    do_c()
  }
}

let my_str = "abcd";

match my_str {
  "abc" => do_sth(),
  "abcd" => do_sth_else(),
  _ => (),
}

Gleam

Similar to Rust’s match, Gleam has case:

type MyEnum {
  A(Int)
  B
  C
}

let x = A(10)

case x {
  A(n) -> do_a(n)
  B -> do_b()
  C -> {
    do_sth()
    do_c()
  }
}

Modules

Rust

In Rust, the mod keyword allows to create a module. Multiple modules can be defined in a single file.

Rust uses the use keyword to import modules, and the :: operator to access properties and functions inside.

mod foo {
    pub fn identity(x: u64) -> u64 {
        x
    }
}

mod bar {
    use super::foo;

    fn main() {
        foo::identity(1);
    }
}

Gleam

In Gleam, each file is a module, named by the file name (and its directory path). Since there is no special syntax to create a module, there can be only one module in a file.

Gleam uses the import keyword to import modules, and the dot . operator to access properties and functions inside.

//// In module foo.gleam
pub fn identity(x) {
  x
}

// in module main.gleam
import foo // if foo was in a folder called `lib` the import would be `lib/foo`
pub fn main() {
  foo.identity(1)
}