From b2cfe8a9d18d658fc54139721a42f500c9520e23 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Mon, 13 Jan 2025 01:46:17 +0000
Subject: monads: Add sequence.

* ravanan/work/monads.scm (sequence): New function.
(state-sequence): New public function.
---
 ravanan/work/monads.scm | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/ravanan/work/monads.scm b/ravanan/work/monads.scm
index 4370ef3..830e7ce 100644
--- a/ravanan/work/monads.scm
+++ b/ravanan/work/monads.scm
@@ -39,6 +39,7 @@
             state-return
             state-let*
             state-begin
+            state-sequence
             current-state
             set-current-state
             run-with-state))
@@ -71,6 +72,19 @@
       (lambda _
         body ...)))))
 
+(define (sequence monad-type mvalues)
+  "Convert a list of monadic @var{mvalues} in @var{monad-type} into a monadic list
+of values."
+  (let ((return (monad-return monad-type)))
+    (mlet* monad-type ((reverse-list
+                        (fold (lambda (mvalue mresult)
+                                (mlet* monad-type ((value mvalue)
+                                                   (result mresult))
+                                  (return (cons value result))))
+                              (return (list))
+                              mvalues)))
+      (return (reverse reverse-list)))))
+
 (define-immutable-record-type <maybe>
   (maybe value valid?)
   maybe?
@@ -226,6 +240,9 @@ maybe-monadic."
   (mbegin %state-monad
     body ...))
 
+(define state-sequence
+  (cut sequence %state-monad <>))
+
 (define (current-state)
   "Return the current state as a state-monadic value."
   (lambda (state)
-- 
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