# Mutation: changing something already there

[Concepts explained](README.md)

**Mutation** means changing an existing value's contents. A **mutable** collection allows this. An **immutable** value does not allow its contents to be edited in place.

Pliro lists and maps can be changed. Text values are immutable: you can give a variable different text, but you cannot replace a character by assigning to a text index.

## Two names, one list

```pliro
# language: en
let scores = [2, 5]
let other = scores
set other[0] = 9
say scores[0]
```

The answer is `9`. Both names refer to the same list. The assignment did not copy its contents into a separate list. This shared name is sometimes called an **alias**.

## Replacing a name's value

Changing `other` to a newly written list, such as `set other = [7, 8]`, would make that name refer to the new list. It would not replace the contents of `scores`. Reassigning a variable and changing a shared collection are different operations.

A **shallow copy** makes a new outer collection but may still share collections nested inside it. A **deep copy** also copies those nested collections. Read the [sharing reference](../syntax/collection-sharing.md) before assuming a function has copied your data.

Try predicting what happens if the changed index is `1` instead of `0`. Which list item does each name show afterward?

[Assignment](../syntax/set.md) · [Side effects](side-effects.md)