From 98329e0a3dd4f78b5d815ac3896272ec70904901 Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Thu, 11 Dec 2025 10:28:11 +0100
Subject: Add remaining haskell book exercises

---
 .../24/ParserExercises/src/Base10Integer.hs        | 52 ++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 Haskell-book/24/ParserExercises/src/Base10Integer.hs

(limited to 'Haskell-book/24/ParserExercises/src/Base10Integer.hs')

diff --git a/Haskell-book/24/ParserExercises/src/Base10Integer.hs b/Haskell-book/24/ParserExercises/src/Base10Integer.hs
new file mode 100644
index 0000000..4e8ee3a
--- /dev/null
+++ b/Haskell-book/24/ParserExercises/src/Base10Integer.hs
@@ -0,0 +1,52 @@
+module Base10Integer where
+
+import Control.Applicative
+import Text.Trifecta
+
+-- 2. Write a parser for positive integer values. Don't reuse the preexisting
+-- digit or integer functions, but you can use the rest of the libraries we've
+-- shown you so far. You are not expected to write a parsing library from
+-- scratch.
+--
+-- Hint: Assume you're parsing base-10 numbers. Use arithmetic as a cheap
+-- "accumulator" for your final number as you parse each digit left-to-right.
+
+parseDigit :: Parser Char
+parseDigit = (char '0')
+    <|> (char '1')
+    <|> (char '2')
+    <|> (char '3')
+    <|> (char '4')
+    <|> (char '5')
+    <|> (char '6')
+    <|> (char '7')
+    <|> (char '8')
+    <|> (char '9')
+
+charToDigit :: Char -> Integer
+charToDigit c = case c of
+                    '0' -> 0
+                    '1' -> 1
+                    '2' -> 2
+                    '3' -> 3
+                    '4' -> 4
+                    '5' -> 5
+                    '6' -> 6
+                    '7' -> 7
+                    '8' -> 8
+                    '9' -> 9
+
+base10Integer :: Parser Integer
+base10Integer = do
+    number <- some parseDigit
+    let n = foldl (\acc x -> (acc * 10) + (charToDigit x)) 0 number
+    return n
+
+-- 3. Extend the parser your wrote to handle negative and positive integers.
+-- Try writing a new parser in terms of the one you already have to do this.
+base10Integer' :: Parser Integer
+base10Integer' = do
+    negative <- (char '-' >> (return negate)) <|> (return id)
+    number <- some parseDigit
+    let n = foldl (\acc x -> (acc * 10) + (charToDigit x)) 0 number
+    return $ negative n
-- 
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