Re: finite state machines in guile

[Jim Blandy <jimb@red-bean.com>]

I think you're being a little inconsistent.

Earlier, you wrote this:

    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.

I read this to mean, "The compiler can do anything it wants, as long
as the machine code has the same meaning as the source code, under the
source language's semantics."  A fine principle.

Then you say:

    I feel that Hobbit's job is to translate nicely expressed scheme code
    into nicely expressed C code. If a tail call in scheme is optimisable
    to a jump then Hobbit should try to find an equivalent C tail call that
    is also optimisable to a jump -- because that is basically the nicest
    way to express the equivalent concept in the C language.

Hobbit is a compiler, too, but you seem unwilling to give it the same
freedom you give your C compiler: to do anything it pleases, as long
as it preserves the semantics.  You want it to produce C code that
resembles your Scheme code.


I think that the goal you've set for Hobbit would actually require
Hobbit to be incorrect; here's why.

The C standard is pretty loose about this, but it is widely agreed
that a valid C implementation does not have to optimize tail calls.
On the other hand, Scheme implementations are required to optimize
tail calls.

This, to me, suggests that a Scheme->C compiler is forbidden to
translate Scheme tail calls into C tail calls.  Given that C
compilers are neither required nor expected to optimize tail calls, it
would seem to me that Hobbit has not really remained true to Scheme's
semantics.


In general, I think it's best to give up on the idea of having
readable output from your Scheme->C compiler; it's a big distraction
from doing good compilation.


(Let me say again: I think Hobbit is useful, and a good thing to have
around.  I'm concerned about the tail call issue, but I understand the
choice Hobbit made.)