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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