From 4b5f216ca9e1890c40bbf8053cfed94e3961849f Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Fri, 7 Feb 2014 10:19:53 -0500
Subject: [PATCH] Add the 'ifoldl2' function to Linear.Matrix.

---
 src/Linear/Matrix.hs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/src/Linear/Matrix.hs b/src/Linear/Matrix.hs
index dd89a9f..6603302 100644
--- a/src/Linear/Matrix.hs
+++ b/src/Linear/Matrix.hs
@@ -37,6 +37,7 @@ import qualified Data.Vector.Fixed as V (
   and,
   fromList,
   head,
+  ifoldl,
   length,
   map,
   maximum,
@@ -823,3 +824,33 @@ matmap f (Mat rows) =
   Mat $ V.map g rows
   where
     g = V.map f
+
+
+-- | Fold over the entire matrix passing the coordinates @i@ and @j@
+--   (of the row/column) to the accumulation function.
+--
+--   Examples:
+--
+--   >>> let m = fromList [[1,2,3],[4,5,6],[7,8,9]] :: Mat3 Int
+--   >>>  ifoldl2 (\i j cur _ -> cur + i + j) 0 m
+--   18
+--
+ifoldl2 :: forall a b m n.
+           (Int -> Int -> b -> a -> b)
+        -> b
+        -> Mat m n a
+        -> b
+ifoldl2 f initial (Mat rows) =
+  V.ifoldl row_function initial rows
+  where
+    -- | The order that we need this in (so that @g idx@ makes sense)
+    --   is a little funny. So that we don't need to pass weird
+    --   functions into ifoldl2, we swap the second and third
+    --   arguments of @f@ calling the result @g@.
+    g :: Int -> b -> Int -> a -> b
+    g w x y = f w y x
+
+    row_function :: b -> Int -> Vec n a -> b
+    row_function rowinit idx r = V.ifoldl (g idx) rowinit r
+
+
-- 
2.43.2