From 874ebc4daeda9c8f9fe3c35201be43da57003fbd Mon Sep 17 00:00:00 2001
From: Rose Hogenson <rhogenson@posteo.net>
Date: Tue, 25 Jan 2022 09:17:01 -0800
Subject: Use an exception for loop return.

This makes everything a lot easier, as it handles nested loops
automatically. My brain wave was storing the returned values in a thunk.
---
 csc/loop.csc | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

(limited to 'csc/loop.csc')

diff --git a/csc/loop.csc b/csc/loop.csc
index 7f59f9e..4756e23 100644
--- a/csc/loop.csc
+++ b/csc/loop.csc
@@ -7,8 +7,16 @@
     ; Once more, from the top!
 
 
-    (define (*current-loop-continuation* . args)
-      (error "Can't use return outside of a loop"))
+    (define-record-type <return-exception>
+      (make-return-exception thunk)
+      return-exception?
+      (thunk return-exception-values))
+
+
+    (define-syntax return
+      (syntax-rules ()
+        ((return expr)
+          (raise (make-return-exception (lambda () expr))))))
 
 
     (define-record-type <loop-termination>
@@ -398,19 +406,5 @@
     (define-syntax loop
       (syntax-rules ()
         ((loop clause* ...)
-          (call/cc
-            (lambda (k)
-              (define prev-loop-continuation *current-loop-continuation*)
-              (dynamic-wind
-                (lambda ()
-                  (set! *current-loop-continuation* k))
-                (lambda ()
-                  (loop-aux "variable-clause-collector" (() ()) clause* ...))
-                (lambda ()
-                  (set! *current-loop-continuation* prev-loop-continuation))))))))
-
-
-    (define-syntax return
-      (syntax-rules ()
-        ((return expr)
-          (call-with-values (lambda () expr) *current-loop-continuation*))))))
+          (guard (e ((return-exception? e) ((return-exception-values e))))
+            (loop-aux "variable-clause-collector" (() ()) clause* ...)))))))
-- 
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