From 6269a1345996b96e2194a54c34028ab73d27a004 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hugo=20H=C3=B6rnquist?= <hugo@lysator.liu.se>
Date: Fri, 16 Nov 2018 00:09:13 +0100
Subject: Add fmap instance for state monad.

---
 control/monad/state.scm | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/control/monad/state.scm b/control/monad/state.scm
index 603e387..f7190af 100644
--- a/control/monad/state.scm
+++ b/control/monad/state.scm
@@ -2,8 +2,9 @@
   #:use-module (oop goops)
   #:use-module (ice-9 match)
   #:use-module (control monad)
-  #:re-export (>>= >>))
-  #:export (return-state run-state get put modify))
+  #:use-module (data functor)
+  #:export (return-state run-state get put modify)
+  #:re-export (>>= >> fmap))
 
 
 ;; Alternative implementation of get.
@@ -71,7 +72,11 @@
     ((r s)
      (list '() (proc s)))))
 
+(define-stateful-method ((fmap (f <procedure>) (s <state>)) st-list)
+  (match ((proc s) st-list)
+    ((r st)
+     (list (f r) st))))
+
 (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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