1-29.hs

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