From ef81a4014ba5e58aa86842e9b1161335d306ca98 Mon Sep 17 00:00:00 2001
From: Jan Sucan <jan@jansucan.com>
Date: Sun, 19 Mar 2023 19:16:24 +0100
Subject: 4_b_1 and 4_b_2: Add solutions

---
 README.md     |  4 ++--
 ch04/4_b_2.hs | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)
 create mode 100644 ch04/4_b_2.hs

diff --git a/README.md b/README.md
index 3f1ecff..238d718 100644
--- a/README.md
+++ b/README.md
@@ -72,8 +72,8 @@ more visible in the list the first exercise of a group is in bold italics.
 | 4_a_2          | yes    |      | |
 | 4_a_3          | yes    |      | |
 | 4_a_4          | yes    |      | |
-| **_4_b_1_**    |        | 97   | |
-| 4_b_2          |        | 98   | |
+| **_4_b_1_**    | yes, in 4_b_2 | 97   | |
+| 4_b_2          | yes    | 98   | |
 | 4_b_3          |        |      | |
 | 4_b_4          |        |      | |
 | 4_b_5          |        |      | |
diff --git a/ch04/4_b_2.hs b/ch04/4_b_2.hs
new file mode 100644
index 0000000..ff92cf9
--- /dev/null
+++ b/ch04/4_b_2.hs
@@ -0,0 +1,47 @@
+-- 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
-- 
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