Lists

Lists in Gleam

Lists are ordered collections of values of the same type:

let numbers = [1, 2, 3, 4, 5]
let names = ["Alice", "Bob", "Charlie"]
let empty: List(Int) = []

Lists in Gleam are linked lists -- they are efficient for prepending and pattern matching, but not for random access.

Prepending

You can add an element to the front of a list with [x, ..rest]:

let numbers = [2, 3, 4]
let more = [1, ..numbers]  // [1, 2, 3, 4]

The List Module

The gleam/list module provides many useful functions:

import gleam/list

list.length([1, 2, 3])                    // 3
list.reverse([1, 2, 3])                   // [3, 2, 1]
list.contains([1, 2, 3], 2)               // True
list.append([1, 2], [3, 4])               // [1, 2, 3, 4]
list.flatten([[1, 2], [3, 4]])             // [1, 2, 3, 4]
list.first([1, 2, 3])                     // Ok(1)
list.rest([1, 2, 3])                      // Ok([2, 3])
list.sort([3, 1, 2], int.compare)         // [1, 2, 3]
list.unique([1, 2, 2, 3, 3])              // [1, 2, 3]
list.zip([1, 2], ["a", "b"])              // [#(1, "a"), #(2, "b")]

Recursive Processing

Since Gleam has no loops, you process lists with recursion or library functions:

fn sum(numbers: List(Int)) -> Int {
  case numbers {
    [] -> 0
    [first, ..rest] -> first + sum(rest)
  }
}

Range-Like Behavior

Gleam does not have a range syntax. You can use list.range:

list.range(1, 5)  // [1, 2, 3, 4, 5]

Your Task

Write a function called sum that takes a list of integers and returns their sum using recursion. Print the sum of [1, 2, 3, 4, 5] and the sum of an empty list.