[lmm]

> Do Haskell programmers not create mocks to test external components?

Do we create test substitutes, alternate implementations of the same interfaces? Yes. But dedicated mocking frameworks are crazy. In Haskell-like languages if you want an implementation of interface foo that returns bar when called with baz, you just... write an implementation of interface foo that returns bar when called with baz. If the easiest way to do that in your language is some kind of magical reflection-based framework, something is very wrong with your language.

[mavelikara]

> If the easiest way to do that in your language is some kind of magical reflection-based framework, something is very wrong with your language.

Languages have strengths and weaknesses. Certain tasks are easy in some languages, and certain other tasks are not. Throwing ones hands up and saying "something is very wrong with your language" because one is not familiar with a technique or tooling popular in another language is immature, IMO.

For example, if you explain how Debug.Trace works in Haskell to programmers familiar with Java, and they'd call it crazy.

[lmm]

> Languages have strengths and weaknesses. Certain tasks are easy in some languages, and certain other tasks are not.

Agreed, but we mustn't fall into the fallacy of assuming that means no language can ever be better or worse than another. There are good and bad language design choices, and "an implementation of interface foo that returns bar when called with baz" is not some obscure specialized feature, it's the basics of general-purpose programming.

> Throwing ones hands up and saying "something is very wrong with your language" because one is not familiar with a technique or tooling popular in another language is immature, IMO.

I'm very familiar with the techniques and tooling of mocking frameworks. I do not make these claims lightly.

[mavelikara]

> Agreed, but we mustn't fall into the fallacy of assuming that means no language can ever be better or worse than another. There are good and bad language design choices.

Agreed.

> "an implementation of interface foo that returns bar when called with baz" is not some obscure specialized feature

It isn't some obscure specialized feature in Java either.

    Foo foo = new Foo() {
      public Bar method(Baz baz) {
        return new Bar("bar");
      }
    }

What mocking frameworks do is to provide a DSL to describe behavior of such implementations, use dynamic bytecode generation (not reflection, BTW) to create implementations of the interfaces dynamically, and bind them to simulate various test conditions. What the makes the language "worse" to require or allow doing this?

My Haskell is rusty, but given Haskell psuedocode like:

   main :: IO ()
   main = do
    f <- foo
    if (f == 1) then
      putStrLn "Got 1"
    else
      putStrLn "Didn't get 1"    
   
   
   foo :: IO Int
   -- ...

how would you test that the two branches of main behave appropriately?

This is not a snark; I am truly interested to know how Haskell gets rid of the need to bind alternate implementations of an interface for testing purposes.

[lmm]

> It isn't some obscure specialized feature in Java either.

To the extent that that's true, fine. I'm sure I see a lot of developers writing something along the lines of:

    Foo foo = mock(Foo.class)
    foo.when(method(any())).return(new Bar("bar"));

instead of that, not because they need any of the mocking features as such but because it takes up fewer lines on the screen, particularly when there are more methods in the interface. (Partly a cultural problem of having overly large interfaces rather than a language problem per se, perhaps).

> use dynamic bytecode generation (not reflection, BTW)

How is it not reflection ("the ability of a computer program to examine, introspect, and modify its own structure and behavior at runtime")?

> What the makes the language "worse" to require or allow doing this?

Reflection or code generation means stepping outside the language and its usual guarantees - any time the programmer is forced to do it it's because the language didn't provide a good way to solve the problem within the language itself. It means you can no longer e.g. extract common expressions, because they don't necessarily mean the same thing; if you have some common mock setup code you can't just blindly automatically extract method, you have to think carefully about when the mocks get instantiated and when the expectations are set.

> how would you test that the two branches of main behave appropriately?

> This is not a snark; I am truly interested to know how Haskell gets rid of the need to bind alternate implementations of an interface for testing purposes.

It doesn't - as I said, you still write test implementations of your interfaces. What it does remove the need for is mocking frameworks, which people use in e.g. Java either because implementing the interface the normal way in the language is more effort (not a problem in Haskell), or because they want to test the specific interactions with the object (e.g. "verify that method foo was called twice") because those methods are used for side effects.

Haskell avoids that one by making it easier to represent actions as values; you can use e.g. a free monad to represent actions that will be performed later, so rather than testing that your complex business logic method called deleteUser(userId) on your mock, you instead test that it returns a DeleteUser(userId) value. To a certain extent you can do this in Java too ("the command pattern"), but without higher-kinded types you can't have a standard implementation of e.g. composed commands or standard methods for working with them, so it gets too cumbersome to really do in practice.

Even in Java you wouldn't want to use mocks for testing methods that operate on simple datatypes: to test e.g. a regex find method, you wouldn't pass in mock strings, you'd just pass in real strings and confirm that the results were true or false as expected. A language like Haskell just expands the space of what you can test in the same these-inputs-these-outputs way, by making it easier to represent more things as values.

[mavelikara]

> not because they need any of the mocking features as such but because it takes up fewer lines on the screen

Why is this a bad thing? How is this different from, say, using Template Haskell?

> It means you can no longer e.g. extract common expressions, because they don't necessarily mean the same thing; if you have some common mock setup code you can't just blindly automatically extract method, you have to think carefully about when the mocks get instantiated and when the expectations are set.

I have never found the existence of tests using mocks being a hindrance to refactoring in Java. Can you provide a more specific example?

PS: > How is it not reflection ("the ability of a computer program to examine, introspect, and modify its own structure and behavior at runtime")?

Agree that it is reflection, by that definition.

[lmm]

> Why is this a bad thing?

Because it shows the language could be a lot better. A common, basic task shouldn't be so much easier outside the language (via reflection) than inside it.

> How is this different from, say, using Template Haskell?

It's the same thing. To the extent that people feel the need to use Template Haskell to do basic and common things, something is very wrong with Haskell.

> I have never found the existence of tests using mocks being a hindrance to refactoring in Java. Can you provide a more specific example?

I mean you can't refactor the test itself. Just basic things like: if you do expect(methodOne(matches("someComplexRegex"))) ; expect(methodTwo(matches("someComplexRegex"))), if you try to pull matches("someComplexRegex") out as a variable you'll break your test (you have to make it a function instead). You can't move an expect() above or below another method call without checking to see whether that was the method it was testing. Individually these things are trivial, but they add up to a chilling effect where people don't dare to improve mock-based tests a little as they work on them, so they end up as repetitive code with subtle variations, just like main code would if you never refactored it.