From d07fc9735f40e545edda456e15dbab783c500f6a Mon Sep 17 00:00:00 2001
From: Gabriel Dos Reis <gdr@axiomatics.org>
Date: Sun, 17 Jan 2016 22:53:01 -0800
Subject: subtractIfCan$CancellationAbelianMonoid: return the more principled
 Maybe %

---
 src/algebra/prtition.spad.pamphlet | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

(limited to 'src/algebra/prtition.spad.pamphlet')

diff --git a/src/algebra/prtition.spad.pamphlet b/src/algebra/prtition.spad.pamphlet
index 6fc92821..56acfbda 100644
--- a/src/algebra/prtition.spad.pamphlet
+++ b/src/algebra/prtition.spad.pamphlet
@@ -91,17 +91,18 @@ Partition(): Exports == Implementation where
       zero? n => 0
       x + (subtractIfCan(n,1) :: NNI) * x
  
-    remv(i: PI,x: %): UN ==
-      member?(i,rep x) => per remove(i, rep x)$Rep
-      "failed"
+    remv(i: PI,x: %): Maybe % ==
+      member?(i,rep x) => just per remove(i, rep x)$Rep
+      nothing
  
     subtractIfCan(x, y) ==
       zero? x =>
-        zero? y => 0
-        "failed"
-      zero? y => x
-      (aa := remv(first rep y,x)) case "failed" => "failed"
-      subtractIfCan((aa :: %), per rest rep y)
+        zero? y => just 0
+        nothing
+      zero? y => just x
+      aa := remv(first rep y,x)
+      aa case nothing => nothing
+      subtractIfCan(aa@%, per rest rep y)
  
     powers x ==
       l := rep x
-- 
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