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X-Authentication-Warning: vesuri.Helsinki.FI: lealanko owned process doing -bs Date: Fri, 2 Oct 1998 16:32:11 +0300 (EET DST) From: Lauri Alanko <lealanko@cc.helsinki.fi> X-Sender: lealanko@vesuri.Helsinki.FI Reply-To: Lauri Alanko <la@iki.fi> X-MIME-Autoconverted: from QUOTED-PRINTABLE to 8bit by cygnus.com id GAA25980 Sender: owner-guile@cygnus.com Precedence: bulk Content-Type: TEXT/PLAIN; charset=ISO-8859-1 Content-Length: 1434 On Fri, 2 Oct 1998, Tel wrote: > > And this is of course just another point to prove that C can't be used > > well as a portable assembler.. :( > > The point of this comment is completely lost on me. Your purpose when > compiling a program is to produce binary code that has the same behaviour as > you have described with your C code. You have no business trying to > specify to the compiler exactly where you want each jump -- so long as > the behaviour is guaranteed consistent. But it isn't! Simply enough: a scheme implementation is required, and thus guaranteed, to be properly tail-recursive. A C implementation has no such guarantee, so a straightforward translation of a scheme function call to a C function call combined with an ordinary C implementation does not constitute an R5RS compliant scheme implementation. Actually, reading the C9X draft more closely (as I don't have C89 handy), it almost looks like tail-call optimization of most functions is forbidden [5.1.2.3:§5, 6.1.2.4:§6], though of course an implementation can break these if it can guarantee it does not affect the semantics of the program.. This sort of problem was precisely the motivation behind C--, a language designed to be a portable compiler target language. It has C syntax, but is much closer to the machine level. For example, there is no stack-based calling mechanism, you have to maintain the stack yourself and use jumps. For compilers, this is great. They can produce code that has different behavior than C supports, e.g., tail recursion, exceptions, co-routines, etc. I'm not sure if it's been published yet. Simon Peyton Jones is one of the designers/developers. -- Clifford Beshers Computer Graphics and User Interfaces Lab beshers@cs.columbia.edu Department of Computer Science http://www.cs.columbia.edu/~beshers Columbia University