Re: Catching Break?

[Ian T Zimmerman <itz@transbay.net>]

> Date: Tue, 26 Jan 1999 01:24:35 +0100
> From: Xavier Leroy <Xavier.Leroy@inria.fr>
> X-Gnus-Article-Number: 93   Mon Jan 25 17:11:10 1999
> 
> > Why doesn't this work:
> >  let main() =
> >   Sys.catch_break true; try Unix.kill (Unix.getpid()) Sys.sigint
> >   with Sys.Break -> prerr_endline "CAUGHT!"; exit 0
> >  let _ = main()
> >  This program just prints "Fatal error: Uncaught exception
> > Sys.Break" as if the try block weren't there.  Am I overlooking
> > something really obvious?

>  No, it's fairly subtle, actually.  For various reasons related to
> the Caml runtime system, signals in OCaml are not necessarily
> handled at the program point where they are received: the signal
> handler is called only at the next "safe" program point.
>  In the case of the ocamlc bytecode interpreter, "safe" program
> points are at function application and at the beginning of each loop
> iteration.  So, in your example above, we leave the "try..with"
> before the handler for the signal is called, and that handler thus
> raises the Sys.Break exception outside of the "try..with".

I understand the kind of consideration which leads to deferring the
handler.  I may be in the middle of a garbage collection etc.

>  If you add a function call after the Unix.kill, everything should
> work as expected

When I have read your mail I thought this would be trivial to work
around.  But it isn't.  First it isn't that obvious what counts as
a function application.  Given the functional nature of ML I'd like to
say "everything".  But then I start to have doubts.  What about basic
arithmetic operators for instance?

let suicide() =
    Unix.kill (Unix.getpid()) Sys.sigint

let suicidal = try 
    begin suicide(); ~-1 end
with Sys.break -> 0

Or how about an assertion?  Is "assert" just a core library symbol, or
a keyword?

let suicidal = try
    begin suicide(); assert true end
with Sys.break -> ()

Finally, and more seriously, this is just a toy example.  In my real
program, I need to return a _value_ from the expression that
corresponds to suicide().  I tried

let suicide() =
    begin Unix.kill (Unix.getpid()) Sys.sigint; 1 end

let id x = x

let suicidal = try
    (suicide(), id 0)
with Sys.break -> (0, 0)


and then

let suicidal = try
    id (suicide())
with Sys.break -> 0


Neither works the way I need, although in both cases there is (in my
naive opinion) a function application between the signal and the end
of the try block.

Do I really have to use _sequencing_ to force a `safe point'?  That
throws away the value from suicide(), so I'll have to invent a
reference to assign to - eeek!

-- 
Ian T Zimmerman                        <itz@transbay.net>
I came to the conclusion that what was wrong about the guillotine
was that the condemned man had no chance at all, absolutely none.
Albert Camus, _The Outsider_