[Haskell-cafe] semantics of concurrent program depends on -O level, -f[no-]omit-yields

[Johannes Waldmann]

> IMHO there is nothing very surprising here: You have 2 threads with no
> synchronization between them whatsoever, so you get what you deserve:
> Undefined behavior. :-) 

Well, yes.

It feels as if the scheduler is mighty unfair here
(delaying the printing indefinitely)
but apparently it is allowed to do so - mainly since there is
no specification that would require otherwise.

But then (seconding your question) what guarantees *do* we have?
For a single-threaded program, it would certainly not be OK
to execute "main = print ()" as "block immediately"?
But when we  forkIO  this, then it can happen?

Possibly related: discussion about (state of formal
specification of) GHC RTS memory model at
https://mail.haskell.org/pipermail/ghc-devs/2018-November/016583.html

- J.W.