[Haskell-beginners] variable parser

9 Jan 2010


      Hi,
    I just want to add a new parser for variables but this doesn't seem to
work can anybody tell me what I'm doing wrong?


module ExprSyn where
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
runExpr :: String -> IO ()
runExpr str = case  runParser expr () " "  str of
   Left err -> putStrLn "Error:" >> print err
   Right val -> print val
demo1 = runExpr  "2+5"
data Expr = Mul Expr Expr
         | Div Expr Expr
         | Add Expr Expr
         | Sub Expr Expr
         | Val Float
  |Let String Expr Expr
  | Var String
 deriving (Eq,Show)
expr :: Parser Expr
expr = variable <|> 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
     <|> variable
     <?> "simple expression"
variable :: Parser Expr
variable = do { b <- letter
       ; do { cs <- word
       ;return (c:cs)
  <|> return [c]
  <|> number
       }

number :: Parser Expr
number = do{ ds <- many1 digit
          ; return (Val $ read ds)
          }


 John

[Haskell-beginners] variable parser

John Moore