Add Defer.recursiveFn to aid in recursion

[johnynek]

It took me a bit of thinking to implement this, so I think it could be of use. This simplifies writing a recursive function over a Defer[F] instance. This is particularly useful with monad transformers that give you Defer instances, e.g. EitherT[IO, MyError, A]

[johnynek]

we make this a lazy val so it is only evaluated once but only inside the defer below so we want until the first time it is actually need.

[johnynek]

but now that I write that down, I can't see a reason why you would need this. You should only apply self inside the recursion, but that means only when loopFn is called, which we don't do until we return the value, so it should be safe.

[satorg]

Interesting thought, thank you!
If I'm getting it correctly, recursiveFn looks like a generalization of fix in some way,
i.e. having fn: F[A] => F[A] we can probably assume that

fix(fn) <=> recursiveFn[Unit, A] { u2fa => Function.const[F[A], Unit](u2fa(())) }(())

[satorg]

Great, thanks!

[johnynek]

I think your law is correct, but also hard to test. Most randomly generated random functions won't terminate I think (or at least many won't). So I don't know if we can actually make that test.