[RFC PATCH 1/2] Add reader extension for interpolated strings.

[Olivier Dion]

From: Olivier dion <olivier-dion@proton.me>

Interpolate strings of the form #"The string @(eval this)".

When no interpolation is required, simply return the raw string.  For
example, #"foo" will return "foo".

When interpolation is required, return an expression that format the
string with the embedded expressions withing it.  For example
#"1+1=@(+ 1 1)" will return (format #f "1+1=~a" (+ 1 1)).

* module/ice-9/boot-9.scm: Extend read hash for strings interpolation.
---
 module/ice-9/boot-9.scm | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/module/ice-9/boot-9.scm b/module/ice-9/boot-9.scm
index a46145ed5..5c0f2eef2 100644
--- a/module/ice-9/boot-9.scm
+++ b/module/ice-9/boot-9.scm
@@ -2254,6 +2254,43 @@ name extensions listed in %load-extensions."
                         (error
                          "#. read expansion found and read-eval? is #f."))))
 
+
+(letrec ((interpolate
+          (lambda (port chars exps)
+            (let ((char (read-char port)))
+              (cond
+               ((eof-object? char)
+                (values
+                 (reverse-list->string chars)
+                 (reverse exps)))
+               ((char=? char #\@)
+                (let ((ahead (peek-char port)))
+                  (cond
+                   ((eof-object? ahead)
+                    (interpolate port chars exps))
+                   ((char=? ahead #\@)
+                    (read-char port)
+                    (interpolate port
+                                 (cons #\@ chars)
+                                 exps))
+                   (else
+                    (interpolate port
+                                 (cons #\a (cons #\~ chars))
+                                 (cons (read port) exps))))))
+               (else
+                (interpolate port (cons char chars) exps)))))))
+  (read-hash-extend #\"
+                    (lambda (char port)
+                      (unread-char char port)
+                      (call-with-values
+                          (lambda () (call-with-input-string (read port)
+                                  (lambda (port)
+                                    (interpolate port '() '()))))
+                        (lambda (fmt args)
+                          (if (null? args)
+                              fmt
+                              `(format #f ,fmt ,@args)))))))
+
 
 
 ;;; {Low Level Modules}
-- 
2.38.1