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+-- 1. Use a fold (choosing the appropriate fold will make your code much
+--    simpler) to rewrite and improve upon the asInt function from the earlier
+--    section called "Explicit recursion" on page 85.
+--
+--      -- file: ch04/ch04.exercises.hs
+--      asInt_fold :: String -> Int
+--
+-- 2. Your function should behave as follows:
+--
+--      ghci> asInt_fold "101"
+--      101
+--      ghci> asInt_fold "-31337"
+--      -31337
+--      ghci> asInt_fold "1798"
+--      1798
+
+import Data.Char (digitToInt)
+
+-- If we used foldr, it would have needed to reconstruct position of the digit
+-- processed in the step function (here nextDigit). When digits are processed
+-- from right to left, we need to know position of the digit in the number so we
+-- can multiply it by the corresponding power of 10 before adding it to the
+-- accumulator. The information about the position would have to be gotten from
+-- a value already in the accumulator.
+--
+-- With foldl, when digits are processed from left to right, the information
+-- about the position is not needed. We just multiply the accumulator by ten to
+-- shift the decimal value to the left, thus making room for one more digit, and
+-- then add value of that digit to the accumulator.
+
+asInt_fold :: String -> Int
+asInt_fold ('-':xs) = -1 * (asInt_fold' xs)
+asInt_fold xs = asInt_fold' xs
+
+asInt_fold' :: String -> Int
+asInt_fold' xs = foldl nextDigit 0 xs
+  where nextDigit acc digit = (acc * 10) + (digitToInt digit)
+
+-- ghci> :l 4_b_2.hs
+-- [1 of 1] Compiling Main             ( 4_b_2.hs, interpreted )
+-- Ok, one module loaded.
+-- ghci> asInt_fold "101"
+-- 101
+-- ghci> asInt_fold "-31337"
+-- -31337
+-- ghci> asInt_fold "1798"
+-- 1798