tasks a-s

Task/S-Expressions/Haskell/s-expressions.hs

@@ -0,0 +1,49 @@
+import Text.ParserCombinators.Parsec ((<|>), (<?>), many, many1, char, try, parse, sepBy, choice)
+import Text.ParserCombinators.Parsec.Char (noneOf)
+import Text.ParserCombinators.Parsec.Token (integer, float, whiteSpace, stringLiteral, makeTokenParser)
+import Text.ParserCombinators.Parsec.Language (haskellDef)
+
+
+data Val = Int Integer
+                   | Float Double
+                   | String String
+                   | Symbol String
+                   | List [Val] deriving (Eq, Show)
+
+lexer = makeTokenParser haskellDef
+
+tInteger = (integer lexer) >>= (return . Int) <?> "integer"
+
+tFloat = (float lexer) >>= (return . Float) <?> "floating point number"
+
+tString = (stringLiteral lexer) >>= (return . String) <?> "string"
+
+tSymbol = (many1 $ noneOf "()\" \t\n\r") >>= (return . Symbol) <?> "symbol"
+
+tAtom = choice [try tFloat, try tInteger, tSymbol, tString] <?> "atomic expression"
+
+tExpr = do
+    whiteSpace lexer
+    expr <- tList <|> tAtom
+    whiteSpace lexer
+    return expr
+    <?> "expression"
+
+tList = do
+    char '('
+    list <- many tExpr
+    char ')'
+    return $ List list
+    <?> "list"
+
+tProg = many tExpr <?> "program"
+
+p ex = case parse tProg "" ex of
+              Right x -> putStrLn $  unwords $ map show x
+              Left err -> print err
+
+main = do
+    let expr = "((data \"quoted data\" 123 4.5)\n  (data (!@# (4.5) \"(more\" \"data)\")))"
+    putStrLn $ "The input:\n" ++ expr ++ "\n"
+    putStr "Parsed as:\n"
+    p expr