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Subject: Re: How to tame compiler?
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* Stefan Monnier <monnier@iro.umontreal.ca> [2021-05-01 06:50]:
> > Independent from the question whether your usage of `eval' is good or
> > valid - there must be some real problem here: if the compiler tells that
> > the lexical variables are unused, their values will not be available in
> > you `eval' call - you would have to create dynamical bindings for that.
> 
> I know I sound like a broken clock, but I think a better answer is to
> avoid `eval`: instead of taking expressions (that you'd pass to `eval`)
> arrange to receive functions (which you'd pass to `funcall` or `apply`).
> Then you can pass those functions the data they need (e.g. the value of
> things like `unsubscribe-text`).

  I have to understand it in this example:
  
  The HTML template is following and it is string:

  <html>
    <head>
      <title>⟦ xml-escape title ⟧</title>
    </head>
    <body>
      <p>Today is one US dollar this many euros: ⟦ usd-eur 1 ⟧</p>
    </body>
  </html>

> I know I sound like a broken clock, but I think a better answer is to
> avoid `eval`: instead of taking expressions (that you'd pass to `eval`)
> arrange to receive functions (which you'd pass to `funcall` or `apply`).
> Then you can pass those functions the data they need (e.g. the value of
> things like `unsubscribe-text`).

  Does that mean I should now extract maybe function `xml-escape' and use apply with rest of arguments?

  Maybe instead of:
  
  <title>⟦ xml-escape title ⟧</title>

  I should make:

  <title>⟦ xml-escape title ⟧</title>

  And then apply xml-escape with "title".

Then I would convert first word into function, that somehow
works, but all the remaining parts of string still need to be
converted. How is program to know that things in a string are for
example other variables or other Lisp functions?

(let* ((lisp (buffer-substring-no-properties
	      (1+ (match-beginning 0)) (1- (match-end 0))))
       (lisp (split-string lisp))
       (function (pop lisp)) 
       (_ (message "%s" function))
       (value (apply (intern function) lisp)) ;; I am stuck here
       (value (string-or-empty-string value)))


For simplest use case like converting string to variable, or something like:

⟦ var hello-name ⟧

I think that would work, but then if I only use variables, I
would not need "var" there, I could just interpolate it into
variable without eval.

For the case where there is even slightly complex Lisp like ⟦ (usd-eur (gold-price-kg)) ⟧
I would not know what to do there.

Maybe it could look like:

⟦ usd-eur (gold-price-kg) ⟧

But then again program would receive string "(gold-price-kg)" and
how would I know that string is another function? I would need to
extract that string and basically make small Lisp reader, again
we come to kind of eval-ing it.

I want to know if there is way to do it.

-- 
Jean

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