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From: adam@gec-mi-at.co.uk (Adam Quantrill)
Newsgroups: sci.lang,comp.lang.misc
Subject: Re: Using C to describe itself
Message-ID: <573@gec-mi-at.co.uk>
Date: Tue, 12-May-87 04:23:43 EDT
Article-I.D.: gec-mi-a.573
Posted: Tue May 12 04:23:43 1987
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In article <7360@boring.mcvax.cwi.nl> steven@cwi.nl (Steven Pemberton, or try steven@mcvax.uucp) writes:
>In article <1403@uwmacc.UUCP> edwards@unix.macc.wisc.edu.UUCP (mark edwards) writes:
>>   Yes indeed C can describe itself.
>
>Actually, I'm not completely convinced of this. The question of whether C
>can be used to describe itself [] Ritchie built a
>trojan horse into the C compiler. The compiler looked identical to its
>previous self, but actually compiled a (slightly) different language. This
>
>[] you can't look at the compiler source to determine what
>a certain construct in C means, even if you know the meaning of the language
>it compiles to.
>Steven Pemberton, CWI, Amsterdam; steven@cwi.nl or try steven@mcvax.uucp

Yes, but I hold that any language that can parse itself, can describe itself.
A non-rigorous argument follows:

1. If the language L can parse itself, it can break down any program into
L primitives.

2. L can implement L primitives.

3. So L can implement any program in L, thus it can describe L given a suitable
parser program.

This is defining L in terms of implementation, not a theoretical description.

As C can parse itself (evidence - cc) I reckon there's a good case that it
can describe itself. The 'trojan horse' compiler mentioned above may be
able to implement 'trojan horse' C, but not C.
       -Adam.

/* If at first it don't compile, kludge, kludge again.*/