Parsec has no default function to parse “any symbolic” string, but it can be added simply by defining an operator new token.

operator = do
  c <- Tok.opStart emptyDef
  cs <- many $ Tok.opLetter emptyDef
  return (c:cs)

Using the expression parser we can extend our table of operators with the “binop” class of custom operators. Note that this will match any and all operators even at parse-time, even if there is no corresponding definition.

binops = [[binary "*" Ex.AssocLeft,
          binary "/" Ex.AssocLeft]
        ,[binary "+" Ex.AssocLeft,
          binary "-" Ex.AssocLeft]
expr :: Parser Expr
expr =  Ex.buildExpressionParser (binops ++ [[binop]]) factor

The parser extension is straightforward and essentially a function definition with a few slight changes. Note that we capture the string value of the operator as given to us by the parser.

binarydef :: Parser Expr
binarydef = do
  reserved "def"
  reserved "binary"
  o <- op
  prec <- int
  return $ BinaryDef o args body

Now for our binary operator, instead of failing with the presence of a binary operator not declared in our binops list, we instead create a call to a named “binary” function with the operator name.

cgen (S.BinaryOp op a b) = do
  case Map.lookup op binops of
    Just f  -> do
      ca <- cgen a
      cb <- cgen b
      f ca cb