How to mock functions from Elixir built-in modules

[tfwright]

@krasenyp

Mocks are very tricky because they test interaction. In your case this is the call to File.read. If you at some point, hypothetically, use a different function for reading, your test will be broken.

This is not necessarily true of Mox at least, it’s part of the “mocks as nouns” rule. By creating your own mock API that is used locally (in the specific logic you want to mock) rather than globally you create more intentional boundaries that are less fragile.