Using when/unless without saving the result of a Monadic action

Question

Is there a way to write this without the variable x?

foo = do
  x <- checker bar
  when x dostuff

I'm imagining something similar to LambdaCase:

foo' = do
  checker bar >>= \case
    True -> dostuff
    _    -> return ()

but without the second case pattern, obviously.

Answer 1

In Control.Monad.Extra, check out whenM. For your example, that would be:

foo = whenM (checker bar) doStuff

Here's a longer example, just to show you how to multiple statements in your whenM structure.

foo = do
  doStuff1
  doStuff2
  whenM (checker bar) $ do
    doStuff3
    doStuff4
    doStuff5
  doStuff6

Answer 2

The most straightforward answer is to desugar the do by hand, and then see if you can write some code that's equivalent but prettier. Your do desugars to:

checker bar >>= \x -> when x dostuff

So, the answer will have to look like checker bar >>= f, where f is equivalent to

\x -> when x dostuff

Well, that's a lot like partially applying when, right? Except you want to supply the second argument instead of the first, so you need to flip it:

checker bar >>= flip when dostuff