Another alternative to string templates

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In the last post I explored a way to expand a string template that was more readable than the usual format. Today I look at another approach where I use sexp expansions to accomplish the same thing. The idea is to embed lisp expressions and replace them by what they evaluate to.

In emacs-lisp, if we have a command in a string, we can "read" it, and then eval it.

Here we get the user-full-name:

(eval (read "user-full-name"))
John Kitchin

We can use this on variables too.

(setq some-variable "test")
(eval (read "some-variable"))
test

So, if we use a syntax to identify what to replace, we can substitute in the values. Let us try %() as the syntax.

(defun expand-template (s)
  "expand a template containing %() with the eval of its contents"
  (replace-regexp-in-string "%(\\([^)]+\\))"
                            (lambda (arg)
                              (format "%s" (eval (read (substring arg 2 -1))))) s))


(let ((key "kitchin-2014")
      (author "Kitchin, J. R.")
      (journal "HACS")
      (year "2014")
      (volume "1")
      (pages "1--10")
      (doi "10.1.1.109/hacs.1.10")
      (url "http://hacs.org/10.1.1.109/hacs.1.10")
      (pdf-dir "/home/jkitchin/pdfs")
      (template "
 :PROPERTIES:
  :Custom_ID: %(key)
  :AUTHOR: %(author
  :JOURNAL: %(journal)
  :YEAR: %(year)
  :VOLUME: %(volume)
  :PAGES: %(pages)
  :DOI: %(doi)
  :URL: %(url)
 :END:
[[cite:%(key)]] [[file:%(pdf-dir)/%(key).pdf][pdf]]\n\n"))

(expand-template template))
 :PROPERTIES:
  :Custom_ID: kitchin-2014
  :AUTHOR: Kitchin, J. R.
  :YEAR: 2014
  :VOLUME: 1
  :PAGES: 1--10
  :DOI: 10.1.1.109/hacs.1.10
  :URL: http://hacs.org/10.1.1.109/hacs.1.10
 :END:
[[cite:kitchin-2014]] [[file:/home/jkitchin/pdfs/kitchin-2014.pdf][pdf]]

That is pretty nice. I like it better than the plist expansion I used before. Presumably these variables would already be defined somewhere in your code.

I thought of trying this on a more complex expansion, and discovered a weakness in the regexp that finds the expansion values. It turns out to be simpler to use %{} as the delimiter than %(), because you may want nested parentheses. The regexp above does not correctly match sets of parentheses.

(defun expand-template (s)
  "expand a template containing %{} with the eval of its contents"
  (replace-regexp-in-string "%{\\([^}]+\\)}"
                            (lambda (arg)
                              (let ((sexp (substring arg 2 -1)))
                                (format "%s" (eval (read sexp))))) s))

(expand-template "2 * 2 = %{(* 2 2)}")
2 * 2 = 4

I am not sure this is a desirable way to make a template, with multiline code to be expanded, but at least this works!

(defun expand-template (s)
  "expand a template containing %{} with the eval of its contents"
  (replace-regexp-in-string "%{\\([^}]+\\)}"
                            (lambda (arg)
                              (let ((sexp (substring arg 2 -1)))
                                (format "%s" (eval (read sexp))))) s))

(expand-template "The result is %{(progn
  (if (> 4 3)
      'true
    'false))}")
The result is true

The regexp used in the expansion is not very robust. In particular if there is a } in the code, it will probably fail because the regexp does not match closing } correctly. Fixing that is beyond me right now!

Copyright (C) 2014 by John Kitchin. See the License for information about copying.

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