[Haskell-cafe] Parsec expressions with alphaNum operators

7 Apr 2008

      Hi,

I'm using buildExpressionParser, and I'd like to use alphanumeric
operator characters. I get an (unexpected "a") error though. With a test
string like "-a" if "a" is used in any of the "reservedOpNames". I'm
aiming for the Fortran operators like ".and.".

The listing below may be helpful. It's taken from the Haskell wiki's
"Parsing expressions and statements" article (minus the statement
part).I've added an ":a:" operator. The article uses "~" as a unary
operator (I'm heading for +/-). It can be tested with:

$ parseTest exprparser "~a"

-- code begins

module Main where

import Control.Monad(liftM)

import Text.ParserCombinators.Parsec

import Text.ParserCombinators.Parsec.Expr

import Text.ParserCombinators.Parsec.Token

import Text.ParserCombinators.Parsec.Language

data Expr = Var String | Con Bool | Uno Unop Expr | Duo Duop Expr Expr

    deriving Show

data Unop = Not deriving Show

data Duop = And | Iff deriving Show

data Stmt = Nop | String := Expr | If Expr Stmt Stmt | While Expr Stmt

          | Seq [Stmt]

    deriving Show

def = emptyDef{ commentStart = "{-"

              , commentEnd = "-}"

              , identStart = letter

              , identLetter = alphaNum

              , opStart = oneOf "~&=:"

              , opLetter = oneOf "~&=:a"

              , reservedOpNames = ["~", "&", "=", ":=", ":a:"]

              , reservedNames = ["true", "false", "nop",

                                 "if", "then", "else", "fi",

                                 "while", "do", "od"]

              }

TokenParser{ parens = m_parens

           , identifier = m_identifier

           , reservedOp = m_reservedOp

           , reserved = m_reserved

           , semiSep1 = m_semiSep1

           , whiteSpace = m_whiteSpace } = makeTokenParser def

exprparser :: Parser Expr

exprparser = buildExpressionParser table term  "expression"

table = [ [Prefix (m_reservedOp "~" >> return (Uno Not))]

        , [Infix (m_reservedOp "&" >> return (Duo And)) AssocLeft]

        , [Infix (m_reservedOp "=" >> return (Duo Iff)) AssocLeft]

        , [Infix (m_reservedOp ":a:" >> return (Duo Iff)) AssocLeft]

        ]

term = m_parens exprparser

       <|> liftM Var m_identifier

       <|> (m_reserved "true" >> return (Con True))

       <|> (m_reserved "false" >> return (Con False))

play :: String -> IO ()

play inp = case parse exprparser "" inp of

             { Left err -> print err

             ; Right ans -> print ans

             }

-- code ends

Cheers,

Paul Keir

Research Student

University of Glasgow

Department of Computing Science

pkeir@dcs.gla.ac.uk