goto-char support for hexadecimal input

Question

I recently realized something weird regarding usability of goto-char using GNU Emacs:

M-x goto-char
Go to char: 0x1000
Please enter a number (OP note: didn't work)

M-x goto-char
Go to char: (* 4 1024)
Please enter a number (OP note: didn't work)

I didn't have to use such a construct for the last 20 years of GNU Emacs usage, but I now need to deal with binary files using nhexl-mode and I can't find a good way to jump to specific addresses using hexadecimal (bonus: if it could be specified in a link such as org-mode link to a file:hex-address).

I can't imagine this very basic thing such as input a Lisp expression inside a value input is not possible.

Answer 1

[@Shynur has already pointed out, both in a comment and in an answer, the syntax for a hexadecimal number which you can use directly in the goto-char interactive prompt. The following answer is concerned solely with the second part of the question: being able to use a Lisp expression in the interactive prompt.]

How a command gets its input is specified by using an interactive specification. E.g. goto-char has an "N" interactive spec[1]:

n -- Number read using minibuffer.

N -- Numeric prefix arg, or if none, do like code ‘n’.

That's probably general enough for 99.99% of the interactive uses of goto-char (e.g. and FWIW, I have never needed a calculator in order to get the position).

For use in Lisp programs, you can certainly pass whatever complicated expression you want as the argument to goto-char, as long as it returns some position. Here's an example from a recent question:

   ...
   (goto-char (progn 
                  (skip-syntax-forward "^ " (line-end-position))
                  (point)))
   ...

Finally, if you want to be able to have a calculator available, you can follow @Shynur's answer, or you can do something like this:

(defun my/goto-char-with-calculator (res)
   (interactive "XType a Lisp expression that evaluates to a number: ")
   (goto-char res))

The doc string of interactive says:

X -- Lisp expression read and evaluated.

So you can say e.g M-x my/goto-char-with-calculator and type an arbitrary Lisp expression that evaluates to a positive integer, e.g (+ 1000 -992) a.k.a 8 or more usefully (+ (point) 10) to advance 10 character positions past the current point in the buffer.

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[1] goto-char is actually written in C and implemented in editfns.c, so I should have said: goto-char has the equivalent of an "N" interactive spec. It is actually implemented by specifying the function call that will do the argument handling: (goto-char--read-natnum-interactive \"Go to char: \"). But that's a detail...

Answer 2

I can't imagine this very basic thing such as input a Lisp expression inside a value input is not possible.

It is because there is no need to do that. In Emacs, this behavior can be changed easily:

(defun Robert-goto-char ()
  (interactive)
  (funcall #'goto-char (call-interactively 'eval-expression)))

You might put it in your .emacs.

---

Here are 2 examples:

M-x Robert-goto-char
Eval: #x1000

M-x Robert-goto-char
Eval: (* 4 1024)

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Redefining goto-char is not recommended, because behaviors of some Elisp code may depend on this function.

Also see 2.4.15 Primitive Function Type:

we discourage redefinition of primitive functions