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Re: Elisp printer

From: Stefan Monnier
Subject: Re: Elisp printer
Date: Tue, 07 Mar 2017 23:39:55 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

> I was wondering about this a while ago and thinking that maybe it could
> be done without C changes by repurposing the extra arguments to
> make-byte-code.  The idea here would be to let cl-defsubst take a new
> :type argument other than 'vector or 'list, meaning "use
> make-byte-code"; and set the :initial-offset to skip over the stuff
> necessary for the bytecode.

Yes, that can be done fairly easily.  But that doesn't give you callable
functions: it just gives you structs represented by those special
"compiled-function" vectors.

E.g. thunk.el creates its thunks with:

    (defmacro thunk-delay (&rest body)
      "Delay the evaluation of BODY."
      (declare (debug t))
      (let ((forced (make-symbol "forced"))
            (val (make-symbol "val")))
        `(let (,forced ,val)
           (lambda (&optional check)
             (if check
               (unless ,forced
                 (setf ,val (progn ,@body))
                 (setf ,forced t))

so we'd need some way to specify both the function's body and its
"struct" at the same time.  What I was thinking of was something like

    (callable-defstruct thunk
      val forced)

    (defmacro thunk-delay (&rest body)
      "Delay the evaluation of BODY."
      (declare (debug t))
      `(make-thunk (&optional check)
         (if check
           (unless forced
             (setq val (progn ,@body))
             (setq forced t))

The idea would be that `forced` and `val` would be fields of the
"callable-struct" and would be accessible directly from the body of the
function (as well as from the outside via thunk-val and thunk-forced
accessors).  That would require changes to the byte-compiler (mostly in
cconv.el) but it can probably be made to work without significant
changes at the C level.


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