Why does Parsec stop parsing with multiple parser?

Question

I'm trying to write a simple parser in Haskell using Parsec but my input "Hello World" is never correctly parsed.

My code looks like this:

parser = p1 <|> p2

p1 = string "Hello"
p2 = string "Hello World"

If I run it I get the error unexpected whitespace

Answer 1

p1 already consumes the tokens "Hello" and therefore p2 instantly fails because the next token is whitespace.

You could use something like try to reset the consumed tokens.

parser = try p1 <|> p2

p1 = string "Hello"
p2 = string "Hello World"