X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2FRoots%2FSimple.hs;h=2906d95839c69daab27580863d8d61bb5f14ccc6;hb=5c0366134e8e1c12772cb685ac14b70d22d6ffed;hp=44d3d62112d2c4f339fa16b65c143e65ed5bb83a;hpb=fe73028041fe3becce6ce1ff268181d55d54a011;p=numerical-analysis.git

diff --git a/src/Roots/Simple.hs b/src/Roots/Simple.hs
index 44d3d62..2906d95 100644
--- a/src/Roots/Simple.hs
+++ b/src/Roots/Simple.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE RebindableSyntax #-}
+{-# LANGUAGE ScopedTypeVariables #-}
 
 -- | The Roots.Simple module contains root-finding algorithms. That
 --   is, procedures to (numerically) find solutions to the equation,
@@ -8,19 +9,34 @@
 --   where f is assumed to be continuous on the interval of interest.
 --
 
-module Roots.Simple
+module Roots.Simple (
+  bisect,
+  fixed_point,
+  fixed_point_error_ratios,
+  fixed_point_iteration_count,
+  has_root,
+  newtons_method,
+  secant_method,
+  trisect )
 where
 
 import Data.List (find)
+import NumericPrelude hiding ( abs )
+import Algebra.Absolute ( abs )
+import qualified Algebra.Additive as Additive ( C )
+import qualified Algebra.Algebraic as Algebraic ( C )
+import qualified Algebra.Field as Field ( C )
+import qualified Algebra.RealField as RealField ( C )
+import qualified Algebra.RealRing as RealRing ( C )
 
-import Normed
+import Normed ( Normed(..) )
+import qualified Roots.Fast as F (
+  bisect,
+  fixed_point_iterations,
+  fixed_point_with_iterations,
+  has_root,
+  trisect )
 
-import qualified Roots.Fast as F
-
-import NumericPrelude hiding (abs)
-import Algebra.Absolute
-import Algebra.Field
-import Algebra.Ring
 
 -- | Does the (continuous) function @f@ have a root on the interval
 --   [a,b]? If f(a) <] 0 and f(b) ]> 0, we know that there's a root in
@@ -41,11 +57,7 @@ import Algebra.Ring
 --   >>> has_root cos (-2) 2 (Just 0.001)
 --   True
 --
-has_root :: (Algebra.Field.C a,
-             Ord a,
-             Algebra.Ring.C b,
-             Algebra.Absolute.C b,
-             Ord b)
+has_root :: (RealField.C a, RealRing.C b)
          => (a -> b) -- ^ The function @f@
          -> a       -- ^ The \"left\" endpoint, @a@
          -> a       -- ^ The \"right\" endpoint, @b@
@@ -57,7 +69,7 @@ has_root f a b epsilon =
 
 
 -- | We are given a function @f@ and an interval [a,b]. The bisection
---   method checks finds a root by splitting [a,b] in half repeatedly.
+--   method finds a root by splitting [a,b] in half repeatedly.
 --
 --   If one is found within some prescribed tolerance @epsilon@, it is
 --   returned. Otherwise, the interval [a,b] is split into two
@@ -69,17 +81,17 @@ has_root f a b epsilon =
 --
 --   Examples:
 --
---   >>> bisect cos 1 2 0.001
---   Just 1.5712890625
+--   >>> let actual = 1.5707963267948966
+--   >>> let Just root = bisect cos 1 2 0.001
+--   >>> root
+--   1.5712890625
+--   >>> abs (root - actual) < 0.001
+--   True
 --
 --   >>> bisect sin (-1) 1 0.001
 --   Just 0.0
 --
-bisect :: (Algebra.Field.C a,
-           Ord a,
-           Algebra.Ring.C b,
-           Algebra.Absolute.C b,
-           Ord b)
+bisect :: (RealField.C a, RealRing.C b)
        => (a -> b) -- ^ The function @f@ whose root we seek
        -> a       -- ^ The \"left\" endpoint of the interval, @a@
        -> a       -- ^ The \"right\" endpoint of the interval, @b@
@@ -89,14 +101,45 @@ bisect f a b epsilon =
   F.bisect f a b epsilon Nothing Nothing
 
 
+-- | We are given a function @f@ and an interval [a,b]. The trisection
+--   method finds a root by splitting [a,b] into three
+--   subintervals repeatedly.
+--
+--   If one is found within some prescribed tolerance @epsilon@, it is
+--   returned. Otherwise, the interval [a,b] is split into two
+--   subintervals [a,c] and [c,b] of equal length which are then both
+--   checked via the same process.
+--
+--   Returns 'Just' the value x for which f(x) == 0 if one is found,
+--   or Nothing if one of the preconditions is violated.
+--
+--   Examples:
+--
+--   >>> let actual = 1.5707963267948966
+--   >>> let Just root = trisect cos 1 2 0.001
+--   >>> root
+--   1.5713305898491083
+--   >>> abs (root - actual) < 0.001
+--   True
+--
+--   >>> trisect sin (-1) 1 0.001
+--   Just 0.0
+--
+trisect :: (RealField.C a, RealRing.C b)
+       => (a -> b) -- ^ The function @f@ whose root we seek
+       -> a       -- ^ The \"left\" endpoint of the interval, @a@
+       -> a       -- ^ The \"right\" endpoint of the interval, @b@
+       -> a       -- ^ The tolerance, @epsilon@
+       -> Maybe a
+trisect f a b epsilon =
+  F.trisect f a b epsilon Nothing Nothing
+
+
 -- | Find a fixed point of the function @f@ with the search starting
 --   at x0. We delegate to the version that returns the number of
 --   iterations and simply discard the number of iterations.
 --
-fixed_point :: (Normed a,
-                Algebra.Field.C b,
-                Algebra.Absolute.C b,
-                Ord b)
+fixed_point :: (Normed a, Additive.C a, Algebraic.C b, RealField.C b)
             => (a -> a) -- ^ The function @f@ to iterate.
             -> b       -- ^ The tolerance, @epsilon@.
             -> a       -- ^ The initial value @x0@.
@@ -110,9 +153,9 @@ fixed_point f epsilon x0 =
 --   @epsilon@. We delegate to the version that returns the number of
 --   iterations and simply discard the fixed point.
 fixed_point_iteration_count :: (Normed a,
-                                Algebra.Field.C b,
-                                Algebra.Absolute.C b,
-                                Ord b)
+                                Additive.C a,
+                                RealField.C b,
+                                Algebraic.C b)
                             => (a -> a) -- ^ The function @f@ to iterate.
                             -> b       -- ^ The tolerance, @epsilon@.
                             -> a       -- ^ The initial value @x0@.
@@ -130,10 +173,10 @@ fixed_point_iteration_count f epsilon x0 =
 --
 --   This is used to determine the rate of convergence.
 --
-fixed_point_error_ratios :: (Normed a,
-                             Algebra.Field.C b,
-                             Algebra.Absolute.C b,
-                             Ord b)
+fixed_point_error_ratios :: forall a b. (Normed a,
+                             Additive.C a,
+                             RealField.C b,
+                             Algebraic.C b)
                    => (a -> a) -- ^ The function @f@ to iterate.
                    -> a       -- ^ The initial value @x0@.
                    -> a       -- ^ The true solution, @x_star@.
@@ -143,7 +186,7 @@ fixed_point_error_ratios f x0 x_star p =
   zipWith (/) en_plus_one en_exp
   where
     xn = F.fixed_point_iterations f x0
-    en = map (\x -> norm (x_star - x)) xn
+    en = map (\x -> norm (x_star - x)) xn :: [b]
     en_plus_one = tail en
     en_exp = map (^p) en
 
@@ -155,18 +198,19 @@ fixed_point_error_ratios f x0 x_star p =
 --   Examples:
 --
 --   Atkinson, p. 60.
+--
 --   >>> let f x = x^6 - x - 1
 --   >>> let f' x = 6*x^5 - 1
 --   >>> tail $ take 4 $ newton_iterations f f' 2
 --   [1.6806282722513088,1.4307389882390624,1.2549709561094362]
 --
-newton_iterations :: (Algebra.Field.C a)
+newton_iterations :: (Field.C a)
                     => (a -> a) -- ^ The function @f@ whose root we seek
                     -> (a -> a) -- ^ The derivative of @f@
                     -> a       -- ^ Initial guess, x-naught
                     -> [a]
-newton_iterations f f' x0 =
-  iterate next x0
+newton_iterations f f' =
+  iterate next
   where
   next xn =
     xn - ( (f xn) / (f' xn) )
@@ -195,7 +239,7 @@ newton_iterations f f' x0 =
 --   >>> abs (f root) < eps
 --   True
 --
-newtons_method :: (Algebra.Field.C a, Algebra.Absolute.C a, Ord a)
+newtons_method :: (RealField.C a)
                  => (a -> a) -- ^ The function @f@ whose root we seek
                  -> (a -> a) -- ^ The derivative of @f@
                  -> a       -- ^ The tolerance epsilon
@@ -241,17 +285,18 @@ iterate2 f x0 x1 =
 --   Examples:
 --
 --   Atkinson, p. 67.
+--
 --   >>> let f x = x^6 - x - 1
 --   >>> take 4 $ secant_iterations f 2 1
 --   [2.0,1.0,1.0161290322580645,1.190577768676638]
 --
-secant_iterations :: (Algebra.Field.C a)
+secant_iterations :: (Field.C a)
                     => (a -> a) -- ^ The function @f@ whose root we seek
                     -> a       -- ^ Initial guess, x-naught
                     -> a       -- ^ Second initial guess, x-one
                     -> [a]
-secant_iterations f x0 x1 =
-  iterate2 g x0 x1
+secant_iterations f =
+  iterate2 g
   where
   g prev2 prev1 =
     let x_change = prev1 - prev2
@@ -266,6 +311,7 @@ secant_iterations f x0 x1 =
 --   Examples:
 --
 --   Atkinson, p. 67.
+--
 --   >>> let f x = x^6 - x - 1
 --   >>> let Just root = secant_method f (1/10^9) 2 1
 --   >>> root
@@ -273,7 +319,7 @@ secant_iterations f x0 x1 =
 --   >>> abs (f root) < (1/10^9)
 --   True
 --
-secant_method :: (Algebra.Field.C a, Algebra.Absolute.C a, Ord a)
+secant_method :: (RealField.C a)
                  => (a -> a) -- ^ The function @f@ whose root we seek
                  -> a       -- ^ The tolerance epsilon
                  -> a       -- ^ Initial guess, x-naught