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04/08: monads: Micro-optimize 'foldm'.


From: David Craven
Subject: 04/08: monads: Micro-optimize 'foldm'.
Date: Fri, 6 Jan 2017 11:16:03 +0000 (UTC)

dvc pushed a commit to branch wip-build-systems-gexp
in repository guix.

commit a15357c743c7384cc18685d79acdb65db4f57c53
Author: Ludovic Court├Ęs <address@hidden>
Date:   Sat Apr 4 17:58:41 2015 +0200

    monads: Micro-optimize 'foldm'.
    
    * guix/monads.scm (foldm): Hoist 'monad-bind' and 'monad-return' calls
      outside of the loop.  Use explicit >>= instead of 'mlet*', and
      explicit return instead of 'with-monad' + 'return'.
---
 guix/monads.scm |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/guix/monads.scm b/guix/monads.scm
index 0b0ad23..8a0f974 100644
--- a/guix/monads.scm
+++ b/guix/monads.scm
@@ -266,7 +266,9 @@ MONAD---i.e., return a monadic function in MONAD."
   (foldm %state-monad (lift2 cons %state-monad) '() '(a b c))
   => '(c b a)  ;monadic
 "
-  (with-monad monad
+  ;; Hoist access to MONAD's 'bind' and 'return' fields outside of the loop.
+  (let ((>>=    (monad-bind monad))
+        (return (monad-return monad)))
     (let loop ((lst    lst)
                (result init))
       (match lst



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