Re: [Caml-list] Compiler killer code?

[William Lovas <wlovas@stwing.upenn.edu>]

On Sat, Jun 08, 2002 at 07:34:02AM -0700, William Lee Irwin III wrote:
> > Is the type inference algorithm guaranteed to termintate? I 
> > ask because I have accidentally attempted to evaluate a 
> > (meaningless) function which almost hanged my Athlon.
> > The killer code is the following:
> > type t = t -> t
> > let f (x:t) :t = f f
> > If I type this into the toplevel, it starts to allocate 
> > memory by the tens of megabytes, until I have to kill it to 
> > prevent a system crash. I let it reach approximately 128MB 
> > before killing it. (BTW, that was with ocaml 3.04+13 
> > 2002-06-05.)
> > Is the language really supposed to accept such garbage as 
> > that which I wrote?

I wonder if it really would have crashed your system, or if O'Caml would
have simply died with an `out of memory' error.

> On Sat, Jun 08, 2002 at 04:32:13PM +0200, Alessandro Baretta wrote:
> 
> I get this instead:
> 
> # type t = t -> t;;
> The type abbreviation t is cyclic

If you use recursive types (`ocaml -rectypes'), it typechecks fine:

    # type t = t -> t;;
    type t = t -> t
    # let rec f (x : t) : t = f f;;
    val f : t = <fun>

In fact, you don't even need to define the type abbreviation:

    # let rec f x = f f;;
    val f : 'a -> 'b as 'a = <fun>

This is O'Caml 3.04 -- if it fails to typecheck in the CVS version, then
i'd say either the semantics of recursive types have changed or a bug has 
been introduced.

cheers,
William
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