# Sharing and copying lists and maps

[Syntax and values](README.md)

Two names can refer to the same list or map. Picture two labels stuck on one box. A second label is an **alias**, not a second box: changing an item through either name changes the shared collection.

A **deep copy** would create separate copies of the box and all collections inside it. Ordinary assignment and passing an argument do not do that. `repeatList` does make independent copies of nested collections for its repeated items. See [changing and sharing values](../concepts/mutation.md).

Assigning or passing a `List` or `Map` copies the value while sharing its
mutable collection storage. Updating an element through one alias is therefore
visible through another alias:

```pliro
# language: en
let original = [0, 0]
let alias = original

set alias[0] = 9
say original  # [9, 0]
```

Use `repeatList` when repeated nested collections must be independent. There is
no general-purpose user-visible deep-copy function in the current library.

## Related entries

[Syntax and values](README.md) · [Built-ins](../builtins/README.md) · [Programming guides](../guides/README.md)