Add remaining haskell book exercises

Haskell-book/24/LearnParsers/app/Main.hs

@@ -0,0 +1,24 @@
+module Main where
+
+import Control.Applicative
+import LearnParsers
+import Text.Fractions
+import Text.Trifecta
+import Text.Parser.Combinators
+
+unitOfSuccess :: (TokenParsing m, Monad m) => m Integer
+unitOfSuccess = do
+    number <- integer
+    _ <- eof
+    return number
+
+type FractionOrNumber = Either Rational Integer
+
+parseFractionOrNumber :: Parser FractionOrNumber
+parseFractionOrNumber =  skipMany (oneOf "\n")
+     >> (Left <$> try virtuousFraction)
+    <|> (Right <$> integer)
+
+main :: IO ()
+main = do
+    print $ parseString unitOfSuccess mempty "123"