Hi John Whilst this won't have the learning value of working through parser combinators yourself, here's code that uses Parsec 2 to do want you want. Its the code from page 12 of Daan Leijen's Parsec manual [1] except it builds a syntax tree of the expression rather than evaluates it. I modified it for a query on Haskell cafe today, but only posted it off list. Some formatting might get "lost in the mail" of course. [1] http://research.microsoft.com/en-us/um/people/daan/download/parsec/parsec.pd... Best wishes Stephen module ExprSyn where import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Expr runExpr :: String -> IO () runExpr str = case runParser expr () "nofile" str of Left err -> putStrLn "Error:" >> print err Right val -> print val demo1 = runExpr "1+1" data Expr = Mul Expr Expr | Div Expr Expr | Add Expr Expr | Sub Expr Expr | Val Integer deriving (Eq,Show) expr :: Parser Expr expr = buildExpressionParser table factor <?> "expression" table :: [[Operator Char st Expr]] table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft] ,[op "+" Add AssocLeft, op "-" Sub AssocLeft] ] where op s f assoc = Infix (do{ string s; return f}) assoc factor :: Parser Expr factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <?> "simple expression" number :: Parser Expr number = do{ ds <- many1 digit ; return (Val $ read ds) } "number"