Add remaining haskell book exercises

2 files changed, 76 insertions, 0 deletions

diff --git a/Haskell-book/24/LearnParsers/src/LearnParsers.hs b/Haskell-book/24/LearnParsers/src/LearnParsers.hs
new file mode 100644
index 0000000..9c349fd
--- /dev/null
+++ b/Haskell-book/24/LearnParsers/src/LearnParsers.hs
@@ -0,0 +1,45 @@
+module LearnParsers where
+
+import Text.Trifecta
+
+stop :: Parser a
+stop = unexpected "stop"
+
+-- read a single character '1'
+one = char '1'
+
+-- read a single character '1', then die
+one' = one >> stop
+-- equivalent to char '1' >> stop
+
+-- read two characters, '1', and '2'
+oneTwo = char '1' >> char '2'
+
+-- read two characters,
+-- '1' and '2', then die
+
+oneTwo' = oneTwo >> stop
+
+testParse :: Parser Char -> IO ()
+testParse p = print $ parseString p mempty "123"
+
+pNL s = putStrLn ('\n' : s)
+
+oneTwoThree :: Parser String
+oneTwoThree = choice
+            [ string "123"
+            , string "12"
+            , string "1"
+            ]
+
+oneTwoThree' = oneTwoThree >> stop
+
+testParse' :: Parser String -> IO ()
+testParse' p = print $ parseString p mempty "123"
+
+oneTwoThree'' :: Parser Char
+oneTwoThree'' = choice
+        [ one
+        , oneTwo
+        , char '1' >> char '2' >> char '3'
+        ]
diff --git a/Haskell-book/24/LearnParsers/src/Text/Fractions.hs b/Haskell-book/24/LearnParsers/src/Text/Fractions.hs
new file mode 100644
index 0000000..f09efc0
--- /dev/null
+++ b/Haskell-book/24/LearnParsers/src/Text/Fractions.hs
@@ -0,0 +1,31 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Text.Fractions where
+
+import Control.Applicative
+import Data.Ratio ((%))
+import Text.Trifecta
+
+badFraction = "1/0"
+
+alsoBad = "10"
+
+shouldWork = "1/2"
+
+shouldAlsoWork = "2/1"
+
+parseFraction :: Parser Rational
+parseFraction = do
+    numerator <- decimal
+    char '/'
+    denominator <- decimal
+    return (numerator % denominator)
+
+virtuousFraction :: Parser Rational
+virtuousFraction = do
+    numerator <- decimal
+    char '/'
+    denominator <- decimal
+    case denominator of
+      0 -> fail "Denominator cannot be zero"
+      _ -> return $ numerator % denominator