Re: How to tame compiler?

[tomas]

On Sun, May 02, 2021 at 10:45:10AM +0300, Jean Louis wrote:
> * Michael Heerdegen <michael_heerdegen@web.de> [2021-05-02 08:43]:
> > Stefan Monnier <monnier@iro.umontreal.ca> writes:
> > 
> > > I know I sound like a broken clock, but [...]
> > 
> > I think you would sound more like an unbroken clock if you would shortly
> > tell the disadvantages of using `eval', then people would be more open
> > to alternatives.
> 
> If possible, let me know one bad example in my context of expanding
> plain text with embedded Emacs Lisp.

I tried to offer an explanation which you chose to ignore [1] . I
think the problem is more subtle, and thus difficult to explain in
just a few words.

If it were that easy to explain clearly, I think Stefan would have
done that already.

> Like how it can go wrong that in same time cannot go wrong in same
> replacement method?
> 
> And which replacement method could I use?
> 
> Like why did Lisp authors the `read-from-string' function if it should
> not be used? It is quite common that its result will be given to `eval'.

It's not as clear-cut as "should not be used". Rather something along
the lines of "should be used with care" [2]. Template expansion is one
of those borderline cases, but if you look at all those modern template
expanders out there [3], you'll realise that they all have some kind of
"custom evaluator", where you explicitly provide an environment, instead
of just punting to `eval' and saying "use... uh, whatever".

For a reason [4].

Cheers

[1] Which, to be honest, is somewhat confusing to me.
[2] Yes, do it: enter the phrase "eval is evil" into your favourite
   search engine.
[3] e.g. Perl's Text::Template, Python's Jinja2, you name it.
[4] Perhaps one short way to express it would be "this havocs
   the compiler". Dunno.

 - t

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