surganov · August 23, 2014 19:26 · Aug 23, 2014
diff --git a/1-29.hs b/1-29.hs
@@ -0,0 +1,32 @@
+cube :: Float -> Float
+cube x = x * x * x
+
+sum' :: (Float -> Float) -> Float -> (Float -> Float) -> Float -> Float
+sum' term a next b =
+    if a > b
+    then 0
+    else term a + sum' term (next a) next b
+
+integral :: (Float -> Float) -> Float -> Float -> Float -> Float
+integral f a b n = (h / 3) * (sum' term 0 (+1) n)  where
+    h = (b - a) / n
+    y k = f $ a + k * h
+    term k
+        | k == 0 || k == n  = y k
+        | odd  k            = 4 * y k
+        | even k            = 2 * y k
+
+main = do
+    print $ integral cube 0 1 100
+
+--  No instance for (Integral Float) arising from a use of `odd'
+--  Possible fix: add an instance declaration for (Integral Float)
+--  In the expression: odd k
+--  In a stmt of a pattern guard for
+--                 an equation for `term':
+--    odd k
+--  In an equation for `term':
+--      term k
+--        | k == 0 || k == n = y k
+--        | odd k = 4 * y k
+--        | even k = 2 * y k