refactor (picrin control list) by following the mannar other monadic

libraries do

contrib/50.for/piclib/for.scm

@@ -2,18 +2,29 @@
   (import (scheme base)
           (picrin control))
 
-  (define-syntax for
+  (define unit list)
+
+  (define (bind m f)
+    (apply append (map f m)))
+
+  (define-syntax reify
     (syntax-rules ()
-      ((_ expr ...)
-       (reset expr ...))))
+      ((_ expr)
+       (reset (unit expr)))))
 
-  (define (in m)
-    (shift k (apply append (map k m))))
+  (define (reflect m)
+    (shift k (bind m k)))
 
-  (define (yield x)
-    (list x))
+  (define zero '())
 
-  (define (null . x)
-    '())
+  (define plus append)
 
-  (export for in yield null))
+  (export unit
+          bind
+          zero
+          plus
+          reify
+          reflect
+          (rename reify for)
+          (rename reflect in)
+          (rename unit yield)))

contrib/50.for/t/test.scm

@@ -4,18 +4,20 @@
 
 (test '(1 2 3)
       (for
-       (yield (in '(1 2 3)))))
+       (in '(1 2 3))))
 
-(test '((1 a) (1 b) (1 c) (2 a) (2 b) (2 c) (3 a) (3 b) (3 c))
+(test '((1 . a) (1 . b) (1 . c) (2 . a) (2 . b) (2 . c) (3 . a) (3 . b) (3 . c))
       (for
        (let ((n (in '(1 2 3)))
              (c (in '(a b c))))
-         (yield (list n c)))))
+         (cons n c))))
 
-(test '((2 a) (2 b) (2 c))
+(define (fail) (in zero))
+
+(test '((2 . a) (2 . b) (2 . c))
       (for
        (let ((n (in '(1 2 3)))
              (c (in '(a b c))))
          (if (even? n)
-             (yield (list n c))
-             (null)))))
+             (cons n c)
+             (fail)))))