[spion]

Its still quite different than classic imperative code

If you're working with a free monad, or if you don't specify IO (just some of the generic IO like typeclasses like say MonadError), you can still choose your own interpreter for the monad and "program" the semicolon. Which means you get back all the benefits of testability etc.

To get a similar effect in an imperative language, you would use e.g. coroutines and `yield` every side effect to the execution engine. The engine will take the action "specs" (probably a data structure describing the action to perform, e.g. set some value in memory) and decide what to do with them, and you can swap the real engine with a test/mock engine in your tests.

[_0w8t]

Programming semicolon is not different from mocking interfaces with imperative code. One still has to write it and tests still do not test the real interfaces. Surely the situation is improved compared with imperative code, but it is not as good as with monad-free code.

It is pity that modern conveniences like polimorphic record types with nice syntax for record updates were not invented earlier. With those even with complex code monads can be used only at the top level when the sugar of do blocks is not even necessarily.