[Haskell] Parsec operator issue

[Greg Fitzgerald]

Hi Adam,

> parse "x=-1", it fails.  Does anyone know how to fix this?

This issue is in using the 'reservedOp' combinator which rejects '='
when followed by '-'.

   reservedOp name =
       lexeme $ try $
       do{ string name
         ; notFollowedBy (opLetter languageDef) <?> ("end of " ++ show name)
         }

If you change:
   reservedOp "="
to:
   char '='
then your test passes.

-Greg


On Tue, Feb 16, 2010 at 2:10 PM, Adam Crume <adamcrume at hotmail.com> wrote:
> I have a parser with a prefix "-" and an infix "=".  When I try to
> parse "x=-1", it fails.  Does anyone know how to fix this?
> I stripped my code down as much as possible.  It parses "x=1" and
> "-1", but fails for "x=-1".
>
>
> import Text.Parsec
> import qualified Text.Parsec.Expr as PE
> import qualified Text.Parsec.Language as L
> import qualified Text.Parsec.Token as T
>
> lexer = T.makeTokenParser L.emptyDef {T.reservedOpNames = ["=", "-"]}
>
> reservedOp = T.reservedOp lexer
>
> integer = T.integer lexer >> return ""
>
> symbol = T.identifier lexer
>
> expression = PE.buildExpressionParser table (integer <|> symbol)
>     where
>          table = [ [ PE.Prefix (reservedOp "-" >> return (\x-> "")) ],
>                    [ PE.Infix (reservedOp "=" >> return (\x-> \y->""))
> PE.AssocRight ] ]
>
> input = do
>   e <- expression
>   eof
>   return e
>
> testParse s = case (parse input "(unknown)" s) of
>                   Left f -> putStrLn $ s ++ "\n" ++ show f
>                   Right f -> putStrLn s
>
> main = do
>   testParse "x=1"
>   testParse "-1"
>   testParse "x=-1"
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