Cristian Baboi wrote: What guarantees that by running the main, the string "Hello world"
will be printed exactly twice ?
The semantics of IO, and the guarantees of the runtime.
IO specifies that (>>) means "compose two actions to make a larger action which does the first actions, then the second action".
[do {a; a;} is notation for a >> a]
The RTS specifies that the "main" action is performed exactly once.
Is this dependent on the implementation (if I use GHC or Hugs) or is something that the language say ?
It's something the language says. IO is part of the runtime, its semantics are defined.
Aside: I tried something like this in WinHugs:
do { xxx<-getLine ; putStrLn xxx }
and pressed two keys at once for the "getLine" action.
The result I've got was an "infinite" loop !!!
If that code loops you have a bug (in hugs?) it certainly shouldn't. It will wait until you press return before it prints anything, though. Jules