[Outreachy] Strategy to implement guix git log --pretty=<string>

[Leo Prikler <leo.prikler@student.tugraz.at>]

Hello Magali,

have you looked into (ice-9 peg)?  An easy pretty grammar, that would
catch your example would be the following:

--8<---------------cut here---------------start------------->8---
(use-modules (ice-9 peg))

(define-peg-pattern commit-hash all (ignore "%h"))
(define-peg-pattern subject all (ignore "%s"))

(define-peg-pattern pretty body
  (* (or commit-hash
         subject
         (* (and (not-followed-by "%") peg-any)))))

(peg:tree (match-pattern pretty "%h %s")) ;; => (commit-hash " "
subject)
--8<---------------cut here---------------end--------------->8---

Of course you have to extend it with all the other percent escapes.
Then you simply map a match-lambda over the "tree", which if given a
symbol returns a string containng the actual value and if given a
string returns the string unchanged.  Finally you string-concatenate
them.  Naturally, you have to check whether the string was valid as
well -- a lone "%" should not match, for instance.

Cheers,
Leo