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Re: Comparing symbol-with-position using eq

From: Paul Eggert
Subject: Re: Comparing symbol-with-position using eq
Date: Fri, 5 Apr 2019 17:50:20 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 4/5/19 4:54 PM, Alan Mackenzie wrote:
> I've been thinking that for the time (nearly 3 years) that I've been
> trying to fix this bug.  Is this how compilers for other Lisp systems are
> written?  It seems horribly easy to compile as Emacs does, by taking the
> (read) starting form and gradually transforming it as a Lisp form.

Sure, it's standard for Lisp compilers to use a representation that is
somewhat more complicated than the original. This kind of practice goes
back a long way. For example, the Multics MACLISP compiler, although it
didn't do a full AST, systematically used a different representation
(i.e., not simple symbols) for variables, a representation that let the
compiler issue more-precise diagnostics. See Bernard Greenberg's
tutorial <https://multicians.org/lcp.html>.

Although this sort of thing does complicate the compiler, that's
typically better than complicating 'eq'. 'eq' is supposed to be verrry
simple and straightforward.

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