From f482ecdb4ec3cd0ecf4698a2917e971aab31f2ac Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@lysator.liu.se>
Date: Fri, 16 Nov 2018 00:08:53 +0100
Subject: Add modify to (control monad state).

---
 control/monad/state.scm | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/control/monad/state.scm b/control/monad/state.scm
index 72a3041..603e387 100644
--- a/control/monad/state.scm
+++ b/control/monad/state.scm
@@ -2,8 +2,8 @@
   #:use-module (oop goops)
   #:use-module (ice-9 match)
   #:use-module (control monad)
-  #:export (return-state run-state get put)
   #:re-export (>>= >>))
+  #:export (return-state run-state get put modify))
 
 
 ;; Alternative implementation of get.
@@ -66,6 +66,11 @@
   "Sets st to v."
   (list '() v))
 
+(define-stateful ((modify proc) st-list)
+  (match st-list
+    ((r s)
+     (list '() (proc s)))))
+
 (define-method (run-state (st <state>) init)
   "Exec state with init as starting state value and st."
   ((proc st) (list init init)))
-- 
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