exprparser.hs

-- File: exprparser.hs
-- This is from the Parsec documentation with minor modification.

module ExprParser where
import Text.ParserCombinators.Parsec
import Text.ParserCombinators.Parsec.Expr
import qualified Text.ParserCombinators.Parsec.Token as P
import Text.ParserCombinators.Parsec.Language

lexer :: P.TokenParser ()
lexer = P.makeTokenParser haskellDef{reservedOpNames = ["*", "/", "+", "-"]}
  
whiteSpace = P.whiteSpace lexer
lexeme = P.lexeme lexer
symbol = P.symbol lexer
natural = P.natural lexer
parens = P.parens lexer
semi = P.semi lexer
identifier = P.identifier lexer
reserved = P.reserved lexer
reservedOp = P.reservedOp lexer

expr :: Parser Integer
expr = buildExpressionParser table factor <?> "expression"

table = [[op "*" (*) AssocLeft, op "/" div AssocLeft]
        ,[op "+" (+) AssocLeft, op "-" (-) AssocLeft]
        ]
        where
          op s f assoc
            = Infix (do { reservedOp s; return f} <?> "operator") assoc

factor = parens expr
         <|> natural
         <?> "simple expression"

runLex :: Show a => Parser a -> String -> IO ()
runLex p input
   = parseTest (do { whiteSpace
                  ; x <- p
                  ; eof
                  ; return x
                  }
              ) input