From 6892f38cf4dab275a91fbe06bfb8efe229518336 Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Tue, 18 Feb 2014 20:25:05 -0500
Subject: [PATCH] Generalize the signature of zipwith2. Implement the dot
 product using zipwith2. Implement matrix multiplication using the dot
 product. Implement the frobenius_norm with zipwith2.

---
 src/Linear/Matrix.hs | 86 ++++++++++++++++++++++++++++++++------------
 1 file changed, 64 insertions(+), 22 deletions(-)

diff --git a/src/Linear/Matrix.hs b/src/Linear/Matrix.hs
index 8266557..8006bde 100644
--- a/src/Linear/Matrix.hs
+++ b/src/Linear/Matrix.hs
@@ -39,7 +39,7 @@ import qualified Data.Vector.Fixed as V (
   toList,
   zipWith )
 import Data.Vector.Fixed.Cont ( Arity, arity )
-import Linear.Vector ( Vec, delete, element_sum )
+import Linear.Vector ( Vec, delete )
 import Naturals ( N1, N2, N3, N4, N5, N6, N7, N8, N9, N10, S, Z )
 import Normed ( Normed(..) )
 
@@ -546,13 +546,12 @@ instance (Ord a,
 --
 infixl 7 *
 (*) :: (Ring.C a, Arity m, Arity n, Arity p)
-        => Mat m n a
-        -> Mat n p a
-        -> Mat m p a
+        => Mat (S m) (S n) a
+        -> Mat (S n) (S p) a
+        -> Mat (S m) (S p) a
 (*) m1 m2 = construct lambda
   where
-    lambda i j =
-      sum [(m1 !!! (i,k)) NP.* (m2 !!! (k,j)) | k <- [0..(ncols m1)-1] ]
+    lambda i j = (transpose $ row m1 i) `dot` (column m2 j)
 
 
 
@@ -567,7 +566,7 @@ instance (Ring.C a, Arity m, Arity n) => Additive.C (Mat m n a) where
   zero = Mat (V.replicate $ V.replicate (fromInteger 0))
 
 
-instance (Ring.C a, Arity m, Arity n, m ~ n) => Ring.C (Mat m n a) where
+instance (Ring.C a, Arity m, Arity n, m ~ n) => Ring.C (Mat (S m) (S n) a) where
   -- The first * is ring multiplication, the second is matrix
   -- multiplication.
   m1 * m2 = m1 * m2
@@ -631,12 +630,13 @@ instance (Absolute.C a,
 --   >>> frobenius_norm m == 3
 --   True
 --
-frobenius_norm :: (Algebraic.C a, Ring.C a) => Mat m n a -> a
-frobenius_norm (Mat rows) =
-  sqrt $ element_sum $ V.map row_sum rows
+frobenius_norm :: (Arity m, Arity n, Algebraic.C a, Ring.C a)
+               => Mat m n a
+               -> a
+frobenius_norm matrix =
+  sqrt $ element_sum2 $ squares
   where
-    -- | Square and add up the entries of a row.
-    row_sum = element_sum . V.map (^2)
+    squares = map2 (^2) matrix
 
 
 -- Vector helpers. We want it to be easy to create low-dimension
@@ -688,7 +688,7 @@ dot :: (Ring.C a, Arity m)
        => Col (S m) a
        -> Col (S m) a
        -> a
-v1 `dot` v2 = unscalar $ ((transpose v1) * v2)
+v1 `dot` v2 = element_sum2 $ zipwith2 (NP.*) v1 v2
 
 
 -- | The angle between @v1@ and @v2@ in Euclidean space.
@@ -834,10 +834,8 @@ ut_part_strict = transpose . lt_part_strict . transpose
 --   15
 --
 trace :: (Arity m, Ring.C a) => Mat m m a -> a
-trace matrix =
-  let (Mat rows) = diagonal matrix
-  in
-    element_sum $ V.map V.head rows
+trace = element_sum2 . diagonal
+
 
 
 -- | Zip together two matrices.
@@ -901,11 +899,11 @@ zip2three m1 m2 m3 =
 --   >>> zipwith2 (^) c1 c2
 --   ((1),(32),(729))
 --
-zipwith2 :: Arity m
-           => (a -> a -> b)
-           -> Col m a
-           -> Col m a
-           -> Col m b
+zipwith2 :: (Arity m, Arity n)
+           => (a -> b -> c)
+           -> Mat m n a
+           -> Mat m n b
+           -> Mat m n c
 zipwith2 f c1 c2 =
   construct lambda
   where
@@ -956,6 +954,18 @@ ifoldl2 f initial (Mat rows) =
     row_function rowinit idx r = V.ifoldl (g idx) rowinit r
 
 
+-- | Left fold over the entries of a matrix (top-left to bottom-right).
+--
+foldl2 :: forall a b m n.
+          (b -> a -> b)
+        -> b
+        -> Mat m n a
+        -> b
+foldl2 f initial matrix =
+  -- Use the index fold but ignore the index arguments.
+  let g _ _ = f in ifoldl2 g initial matrix
+
+
 -- | Fold over the entire matrix passing the coordinates @i@ and @j@
 --   (of the row/column) to the accumulation function. The fold occurs
 --   from bottom-right to top-left.
@@ -1081,3 +1091,35 @@ inverse matrix =
   where
     lambda i j = cofactor matrix i j
 
+
+
+-- | Retrieve the rows of a matrix as a column matrix. If the given
+--   matrix is m-by-n, the result would be an m-by-1 column whose
+--   entries are 1-by-n row matrices.
+--
+--   Examples:
+--
+--   >>> let m = fromList [[1,2],[3,4]] :: Mat2 Int
+--   >>> (rows2 m) !!! (0,0)
+--   ((1,2))
+--   >>> (rows2 m) !!! (1,0)
+--   ((3,4))
+--
+rows2 :: (Arity m, Arity n)
+      => Mat m n a
+      -> Col m (Row n a)
+rows2 (Mat rows) =
+  Mat $ V.map (mk1. Mat . mk1) rows
+
+
+
+-- | Sum the elements of a matrix.
+--
+--   Examples:
+--
+--   >>> let m = fromList [[1,-1],[3,4]] :: Mat2 Int
+--   >>> element_sum2 m
+--   7
+--
+element_sum2 :: (Arity m, Arity n, Additive.C a) => Mat m n a -> a
+element_sum2 = foldl2 (+) zero
-- 
2.43.2