From 0f968fcc28bbb48ad0d65938b05bed3d75c7819c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@hornquist.se>
Date: Wed, 24 Apr 2019 19:40:46 +0200
Subject: Add {mod,set}/r!

`set!' by default has an unspecified return value. But that haven't
stopped me from using that (set! (acc obj) val) returns the value
returned by the internal setter function.

This change requires you to explicitly request the returning version of
set, which returns the value of the last set field.
---
 module/output/text.scm |  4 ++--
 module/util.scm        | 29 +++++++++++++++++++++++++----
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/module/output/text.scm b/module/output/text.scm
index 7c99e12f..274c79e2 100644
--- a/module/output/text.scm
+++ b/module/output/text.scm
@@ -23,7 +23,7 @@
     (string-concatenate/shared
      (merge words (map (lambda (n) (make-string n #\space))
                        space-list)
-            (let ((f #t)) (lambda _ (mod! f not)))))))
+            (let ((f #t)) (lambda _ (mod/r! f not)))))))
 
 
 ;; Splits and justifies the given line to @var{#:width}.
@@ -34,7 +34,7 @@
     (let* ((head tail (take-drop-while
                        (let ((w 0))
                          (lambda (word)    ; Take words until we are above the limit.
-                           (< (mod! w = (+ 1 (string-length word)))
+                           (< (mod/r! w = (+ 1 (string-length word)))
                               width)))
                        lst)))
       (cond ((null? tail) (list (unwords head))) ; Don't justify last line.
diff --git a/module/util.scm b/module/util.scm
index 14b6cfde..9a816cee 100644
--- a/module/util.scm
+++ b/module/util.scm
@@ -6,6 +6,7 @@
                                for-each-in for
                                define-quick-record
                                mod! sort* sort*!
+                               mod/r! set/r!
                                find-min
                                catch-multiple)
   #:replace (let* set! define-syntax)
@@ -20,6 +21,8 @@
     ((_ otherwise ...)
      ((@ (guile) define-syntax) otherwise ...))))
 
+(define-public unspecified (if #f #f))
+
 
 
 (define-public upstring->symbol (compose string->symbol string-upcase))
@@ -95,6 +98,9 @@
 
 
 
+
+
+
 ;; Replace let* with a version that can bind from lists.
 ;; Also supports SRFI-71 (extended let-syntax for multiple values)
 ;; @lisp
@@ -164,10 +170,8 @@
 ;; Still requires all variables to be defined beforehand.
 (define-syntax set!
   (syntax-rules ()
-    ((_ field expr)
-     (let ((val expr))
-       ((@ (guile) set!) field val)
-       val))
+    ((_ field val)
+     ((@ (guile) set!) field val))
     ((_ field val rest ...)
      (begin ((@ (guile) set!) field val)
             (set! rest ...)))))
@@ -179,9 +183,11 @@
     ((_ field proc)
      (set! field (proc field))))  )
 
+
 ;; Like set!, but applies a transformer on the already present value.
 (define-syntax mod!
   (syntax-rules (=)
+    ((_) unspecified)
     ((_ field = proc)
      (modf% field = proc))
 
@@ -194,6 +200,21 @@
     ((_ field proc rest ...)
      (begin (modf% field proc) (mod! rest ...)))))
 
+(define-syntax-rule (set/r! args ... final)
+  (let ((val final))
+    (set! args ... val)
+    val))
+
+(define-syntax mod/r!
+  (syntax-rules (=)
+    ((_ args ... field = (proc pargs ...))
+     (begin (mod! args ...)
+            (set/r! field (proc field pargs ...))))
+    ((_  args ... ffield fproc)
+     (begin (mod! args ...)
+            (set/r! ffield (fproc ffield))))))
+
+
 ;; This function borrowed from web-ics (calendar util)
 (define* (sort* items comperator #:optional (get identity))
   "A sort function more in line with how python's sorted works"
-- 
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