Declaring

Lists are linear collections of elements of the same type. Linear means that, in order to reach an element in a list, we must visit all the elements before (sequential access). Elements can be repeated, as only their order in the collection matters. The first element is called the head, and the sub-list after the head is called the tail. For those familiar with algorithmic data structure, you can think of a list a stack, where the top is written on the left.

💡 Lists are needed when returning operations from a smart contract.

The type for lists is polymorphic, that is, parameterised by the type of the list elements, so we can define a "list of integers", a "list of natural numbers" etc.

const empty_list : list<int> = [];

const my_list : list<int> = [1, 2, 2]; // The head is 1, the tail is [2, 2]

Note how we need to use the cast list(...) on a tuple to make it a list. In general, tuples are not lists: tuples have a fixed number of components that appear in their type, and each component can have a different type, whereas lists have a variable number of elements and they have all the same type. Nevertheless, LIGO uses the same syntax for tuples and lists, except that the latter is enclosed in list(...), except when the context makes it unambiguous that it is a list (we will see some example with pattern matching).

See predefined namespace List.