Choose a collection

Collections hold several values. Choose the type based on how you will read and change those values.

Import: import system.collections;

You need…	Use
Items you can read by position	List
Values you look up by a key	Map
Keys kept in sorted order	SortedMap
Unique values	Set
Items processed in arrival order	Queue
The most recently added item first	Stack
The smallest or highest-priority item first	PriorityQueue
Filtering and transforming items	Queries and Seq

Guides in this section

• ListA growable sequence you read and change by position.API
• MapLook up values by key.API
• SortedMapA map that keeps its keys in order.API
• SetKeep unique values and test membership.API
• QueueProcess items in the order they arrive.API
• StackRemove the most recently added item first.API
• PriorityQueueRemove the smallest or highest-priority item first.API
• Queries and SeqFilter, transform, and combine collection items.API

Start with a list

import system;
import system.collections;

fun main() {
    let names: List<string> = ["Ada", "Sam"];
    names.push("Lee");
    for (let name in names) {
        System.println(name);
    }
}

This prints each name on a separate line. The List<string> type tells Dream to make a growable list. Without that expected type, square brackets create a fixed array.

Typed {value, value} creates a set, and typed {key: value} creates a map. Empty collections need a type, such as let names: Set<string> = {};.

Missing items and positions

Check each collection's result type before assuming an item exists. Map lookups and queue removals can return None. List indexing uses a position and stops the program if that position is outside the list.

See Option and Result for handling absence and the API catalog for all public members.