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Comment by beaconstudios | Hacker News Reader

I find that you can keep with a class-based approach if you utilise your IOC container in your tests, and only swap out side-effectful classes (ORM, remote service clients, etc) with mocks. That way you don't need to concern yourself with the dependencies of the classes you're testing so you can refactor the underlying relationships and objects with impunity as long as you don't change the interfaces of the classes you're testing.

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I don't know if I asked my question as clearly as possible. Mocking your DB, the file system, the remote API, etc are all a given when doing tests. My question was aimed at trying to find out if it's feasible to replace the imperative Presenter/Service/etc patterns with a more functional approach.

What I'd like to see is testing only the returned value of a function, as opposed to calling a void function and checking whether the injected mocks got the proper method invocations, i.e. whether the desired side effects occurred.

if you're calling a void function then by definition you're calling a function that does nothing but side effects or calling off to other classes. In the latter case, they can be good candidates for refactoring in my experience. A function that does something but doesn't either produce mockable side-effects or return information may be a code smell.

A function can return void and still throw an exception. (Like one I wrote today, "insert row into table", which throws on failure and return nothing on success.)

A bunch of functions that all return void all being called in a row, though? That's a much smellier smell. What happens if their order gets mixed up, for example.

Testing that function would be a matter of mocking out the database with a class that can emulate the type of failure that would trigger your exception.

Ooooh, yes. Testing error-handling code is another thing I keep on forgetting to do.

it's almost more worthwhile than testing your happy path - when things go wrong you want to have guarantees that they'll be handled correctly and not kill your server.

Lots of systems separate commands and queries. Commands would be difficult to test without some mocking.

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