String to decimal integer

import Data.Char

-- stdi (string to decimal integer) takes a string of digits and turns
-- it into a decimal integer, sort of like atoi in C.
stdi :: [Char] -> Int
-- The function turns a string into a list (stl), goes through the
-- list multiplying its items by ten to a power of a decrementing
-- value and accumulating the results (lti); it calculates the maximum
-- power value (mpv) by calculating the length of the list and
-- subtracting 1.
stdi str = lti mpv stl
-- mpv - maximum power value: e.g. numbers from 1000 to 9999 consist
-- of 4 digits therefore the maximum power is 4 - 1 = 3
  where mpv = length stl - 1
-- stl - string to list: applies the function digitToInt to every
-- character in string and returns a list of integers.
        stl = map digitToInt str
-- lti goes through the list of integers multiplying them by
-- ten to a power of a decrementing value and accumulating the results
        lti _ [] = 0
        lti n (x:xs) = x * 10 ^ n + lti (n - 1) xs

main :: IO ()
main = print $ stdi "1000000" - 1
-- 999999