Parse Error on Input '<-' inside a do block?

Question

I'm trying to do some parsing in Haskell using Parsec. I've got a number of parsers in my code, but am getting an error on one of them:

expression2 =
    do (operator lexer "|"
        a <- alternate
        as <- expression2
        return $ a:as
  ) <|> return []

The error is parse error on input '<-, on the a <- alternate line.

Can anyone explain why I'm getting this error, and how to fix it?

Thanks in advance.

Answer 1

Did you put the parens in the wrong place?

expression2 =
   (do  operator lexer "|"
        a <- alternate
        as <- expression2
        return $ a:as) <|> return []