How to define a daemon thread of type IO Void?

[istathar]

I'm sure I'm missing something obvious but for a thread that "never returns" fork_ needs an IO Void.

How do you make an IO Void? I mean I know it could be undefined which is of type a but that seems suboptimal!

The only example I could find is in your test suite and you're using throwIO (which is also of type a, of course). Is that what you had in mind, that every daemon thread that "never returns" would end with a throw?

[evanrelf]

You can use forever:

forever :: Applicative f => f a -> f b

For example:

echo :: IO Void
echo = forever do
  input <- getLine
  putStrLn input

[ocharles]

More generally, anything that won't terminate will be a computation of type IO a, which fits the requirement for IO Void

[mitchellwrosen]

Yep, ditto to both of the previous comments. You could use forever as @evanrelf suggests, or if your daemon manually calls itself, either way - just ascribe it either the type IO a or IO Void.

[evanrelf]

I think of it like this:

A function's return type is a promise that you'll return that thing once you're done running.

But if you never finish running, you never have to make good on that promise, so you can make wild claims.

It's impossible to return Void, but you can claim you will once you're done, because you'll never be done 🙂