src/Normed.hs: combine zero- and nonzero-length implementations.

[numerical-analysis.git] / src / Normed.hs

diff --git a/src/Normed.hs b/src/Normed.hs
index f339ebfd6757a86123f8111c4cb1bb7f5ef35534..28fbd2878a2d02641820081da67578ba1166bce3 100644 (file)
--- a/src/Normed.hs
+++ b/src/Normed.hs
@@ -8,18 +8,33 @@ where
 
 import BigFloat
 
-import NumericPrelude hiding (abs)
-import Algebra.Absolute (abs)
-import qualified Algebra.Absolute as Absolute
-import qualified Algebra.Algebraic as Algebraic
-import qualified Algebra.RealField as RealField
-import qualified Algebra.ToInteger as ToInteger
+-- Ensure that we don't use the Lattice.C "max" function, that
+-- only works on Integer/Bool.
+import NumericPrelude hiding ( abs, max )
+import Algebra.Absolute ( abs )
+import qualified Algebra.Absolute as Absolute ( C )
+import qualified Algebra.Algebraic as Algebraic ( C )
+import Algebra.Algebraic ( root )
+import qualified Algebra.RealField as RealField ( C )
+import qualified Algebra.ToInteger as ToInteger ( C )
+import qualified Algebra.ToRational as ToRational ( C )
+import qualified Data.Vector.Fixed as V (
+  Arity,
+  foldl,
+  map )
+import Data.Vector.Fixed.Boxed ( Vec )
+import qualified Prelude as P ( max )
+import Linear.Vector ( element_sum )
 
+
+-- | Instances of the 'Normed' class know how to compute their own
+--   p-norms for p=1,2,...,infinity.
+--
 class Normed a where
   norm_p :: (ToInteger.C c, Algebraic.C b, Absolute.C b) => c -> a -> b
   norm_infty :: (RealField.C b) => a -> b
 
-  -- | The "usual" norm. Defaults to the Euclidean norm.
+  -- | The \"usual\" norm. Defaults to the 2-norm.
   norm :: (Algebraic.C b, Absolute.C b) => a -> b
   norm = norm_p (2 :: Integer)
 
@@ -43,3 +58,34 @@ instance Normed Float where
 instance Normed Double where
   norm_p _ = abs . fromRational' . toRational
   norm_infty = abs . fromRational' . toRational
+
+
+-- | 'Normed' instance for vectors of any length. We will generally be
+--   working with n-by-1 /matrices/ instead of vectors, but sometimes
+--   it's convenient to have these instances anyway.
+--
+--   Examples:
+--
+--   >>> import Data.Vector.Fixed (mk3)
+--   >>> import Linear.Vector (Vec0, Vec3)
+--   >>> let b = mk3 1 2 3 :: Vec3 Double
+--   >>> norm_p 1 b :: Double
+--   6.0
+--   >>> norm b == sqrt 14
+--   True
+--   >>> norm_infty b :: Double
+--   3.0
+--
+--   >>> let b = undefined :: Vec0 Int
+--   >>> norm b
+--   0.0
+--
+instance (V.Arity n, Absolute.C a, ToRational.C a, Ord a)
+         => Normed (Vec n a) where
+  norm_p p x =
+    (root p') $ element_sum $ V.map element_function x
+    where
+      element_function y = fromRational' $ (toRational y)^p'
+      p' = toInteger p
+
+  norm_infty x = fromRational' $ toRational $ (V.foldl P.max 0) $ V.map abs x