Suppose that we have the following parser: p = lookAhead (char 'a') >> char 'b' If we use it like so parse p "" "a" we get the following error: Left (line 1, column 1): unexpected "a" expecting "b" What happened is that char 'a' succeeded by consuming the 'a' from the input and then lookAhead rewinded the input stream (as it does on success). Then, char 'b' tries to parse (again) the first character of the input and fails. Everything works as expected. Now let's slightly modify our parser: p' = lookAhead (many1 $ char 'a') >> char 'b' I've only added a many1. I was expecting this parser to give the same error as the previous one: many1 $ char 'a' will succeed consuming one 'a' and then lookAhead will rewind the input (as it does on success). Thus when we call char 'b' we are going to be in the beginning of the input again. Well, that doesn't happen: Left (line 1, column 2): unexpected end of input expecting "b" As you can see, lookAhead did not rewind the input as it was supposed to.