f-strings in emacs-lisp

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I am a big fan of f-strings in Python 3. They let you put variable names and expressions in a string template that get expanded to create new strings. Here is a simple example of using those:

username = 'John Kitchin'
somevar = 5**0.5
print(f'{username:30s}{somevar:1.2f}')
John Kitchin                  2.24


String formatting in emacs-lisp is by comparison not as fun and easy. Out of the box we have:

(let ((username "John Kitchin")
      (somevar (sqrt 5)))
  (format "%-30s%1.2f" username somevar))
John Kitchin                  2.24

That is still three lines of code, but it is ugly and hard to read like the old python code:

print('%-30s%1.2f' % (username, somevar))
John Kitchin                  2.24


My experience has shown that this gets harder to figure out as the strings get larger, and f-strings are easier to read.

The wonderful 's library provides some salvation for emacs-lisp, if you don't want the format fields. You can refer to variables in a lexical environment like this.

(let ((username "John Kitchin")
      (somevar (sqrt 5)))
  (s-lex-format "${username}${somevar}"))
John Kitchin2.23606797749979

Today, I decided to do something about this, and wrote this little macro. It is a variation on s-lex-format that introduces a slightly new syntax. You can now add an optional format field separated from the variable name by a space.

(defmacro f-string (fmt)
  "Like `s-format' but with format fields in it.
FMT is a string to be expanded against the current lexical
environment. It is like what is used in `s-lex-format', but has
an expanded syntax to allow format-strings. For example:
${user-full-name 20s} will be expanded to the current value of
the variable `user-full-name' in a field 20 characters wide.
  (let ((f (sqrt 5)))  (f-string \"${f 1.2f}\"))
  will render as: 2.24
This function is inspired by the f-strings in Python 3.6, which I
enjoy using a lot.
"
  (let* ((matches (s-match-strings-all"${\\(?3:\\(?1:[^} ]+\\) *\\(?2:[^}]*\\)\\)}" fmt))
         (agetter (loop for (m0 m1 m2 m3) in matches
                        collect `(,m3 .  ,(format (format "%%%s" (if (string= m2 "")
                                                                     (if s-lex-value-as-lisp "S" "s")
                                                                   m2))
                                                  (symbol-value (intern m1)))))))

    `(s-format ,fmt 'aget ',agetter)))
f-string

Here it is in action.

(let ((username "John Kitchin")
      (somevar (sqrt 5)))
  (f-string "${username -30s}${somevar 1.2f}"))
John Kitchin                  2.24

It still lacks some of the capability of f-strings in python, e.g. in Python, arguments inside the template to be expanded get evaluated. The solution used above is too simple for that, since it just used a regexp and is limited to the value of variables in the lexical environment.

print(f'{5**0.5:1.3f}')
2.236


Nevertheless, this simple solution matches what I do most of the time anyway, so I still consider it an improvement!

Copyright (C) 2018 by John Kitchin. See the License for information about copying.

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