`choice` and `or` combinators do not work correctly with `string`

[typemeta]

`choice` and `or` combinators do not work correctly with `string`

[jon-hanson]

Hi, thanks for your comments. The error is due to the fact that the parser is for LL(1) grammars, meaning grammars that can be parsed by only looking 1 character ahead (i.e. at the next char) at each step. So, as you guessed, your grammar breaks this as the two alternative words share the first 2 characters ("to"). This means the parser has to look 3 characters ahead to determine which of the two words it is.

The simplest way to address this is to parse the common prefix first and then parse the remaining suffix as a choice:

public static final Parser<Chr, String> day =
        string("to")
                .andR(choice(
                        string("day").andR(pure("today")),
                        string("morrow").andR(pure("tomorrow"))
                ));

I hope that helps.

[franza]

Okay, now I get it right. Thanks for explanation about LL(1) grammar, I am usually ignoring such info when reading documentation :) Thanks for the help!