Mutation: changing something already there
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
# 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 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?