Chapter4 Task 8 (A conundrum)

[sa-mendez]

First of all, I love the course. I have been dabbling in Haskell over the last few months when I stumbled upon your course, and really appreciate it as a resource.

The Task is as follows:

Can you implement a monad version of AND, polymorphic over any monad?
🕯 HINT: Use "(>>=)", "pure" and anonymous function
-}
andM :: (Monad m) => m Bool -> m Bool -> m Bool

I went ahead and implemented it (wont post solution here) as prescribed, and upon running the tests, only the following test case failed:

it "Just False - Nothing" $ andM (Just False) Nothing shouldBe Just False

Intuitively, this makes sense, because of boolean short circuiting. However, if you run the following in ghci (note that this would be equivalent to a cheating implementation of andM = liftM2 (&&) ):

liftM2 (&&) (Just False) Nothing

You get Nothing, which would also fail the unit test, and is at least consistent with what my implementation outputs.

I did update my implementation to short-circuit and got the test to pass, but given the results of using liftM2, is short-circuiting necessary for correctness when doing boolean AND in a monadic context?

Thank in advance,

Sergio

[vrom911]

The ordinary && is lazy on the second argument. Meaning that if it meets False as the first argument, there is no need to calculate another arg. In andM we would like to keep the same behaviour, so the

Just False `andM` _

should be Just False 🙂

If you don't use the >>= operator or use liftM2 (that have the same behaviour as liftA2 unfortunately), then the Monad constraint is too strong, and it is enough to specify only Applicative. However, with Applicative you are not able to implement a lazier version of andM.

[sa-mendez]

Thank you for clearing that up.

Out of curiosity I went and implemented andM using do notation, and my naive implementation of it suffered from the same problem where it was not lazy on the second argument. I had to purposely introduce short-circuiting in my do notation solution in order for

Just False andM_

to properly yield Just False