"""Pyret makes testing a natural part of the programming process. Functions can end in a where: clause that holds unit tests for the function. These assertions are checked dynamically."""
Fantastic idea! I'll keep that in mind, should be fairly easy to extend Lisps or other AST-Macro enabled languages (Elixir, Julia, Python) with such a functionality. I really like that. It makes it easy to work on a function and code tests down without switching contexts (files, workspaces, etc)
It clutters the code.
Tests are a form of documentation, but too much in the code, like too many comments, obscures. Higher level unit tests (acceptance tests) can be quite long, especially if there's a lot of setup - unlike their example code. Literate programming tried embedded documentation, but didn't catch on (even with Knuth's backing). Embedding tests makes them easier to keep in sync, but tests are already kept in sync by (hopefully) failing when not. However, their idea of automatically running tests is interesting, so there's no infrastructure to set-up, run etc (though you'd want to be able to disable them).
Nice for teaching.
I would argue literate programming is experiencing a renaissance, thanks to docco: