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Immutability

In FP, we love to make data immutable. Immutable values can't be changed after they're created so they're easier to think about.

Say you're trying to fix a bug where a user doesn't have as much money in their account as they should. If twenty different functions all modify that same variable, it's going to be a nightmare to figure out which one is the culprit!

Tuples vs. Lists

Tuples and lists are both ordered collections, but lists are mutable:

ages: list[int] = [16, 21, 30]
ages.append(80)
# [16, 21, 30, 80]

while tuples are immutable:

ages: tuple[int, ...] = (16, 21, 30)
# (16, 21, 30)
more_ages: tuple[int, ...] = (80,)
# (80,)
all_ages: tuple[int, ...] = ages + more_ages
# (16, 21, 30, 80)

Here, all_ages is a new tuple that has the values from ages and more_ages, but the original tuples are unchanged.

Assignment

There's a bug in the add_prefix function! It's supposed to add a prefix to a document, then return a new tuple containing the provided documents and the updated document as the last element.

Fix the bug by adding a comma like this: (new_doc,) so that Python recognizes it as a single-element tuple.