Question: choice not taking alternate paths about funcj

[jon-hanson]

Firstly, you don't need to use both the choice and or combinators at the same time. choice(A, B, C) is a short-hand for A.or(B).or(C). I.e. you can define p either using choice:

final Parser<Chr, String> p = choice(string("128"), string("16"), string("1"));

or using or:

final Parser<Chr, String> p = string("128").or(string("16")).or(string("1"));

(both are equivalent)

Next, your grammar isn't LL(1) because for the rule:

P = "128" | "16" | "1"

we need to look 2 characters ahead when a "1" is encountered - in order to resolve between the 3 rules that begin with a "1". To resolve this you need to restructure the rule so that it first parses the "1" and then attempts to parse the remainder:

P = "1" R
R = "28" | "6" | <eof>

Using the funcj parser this would like something like this:

final Parser<Chr, String> p =
       string("1")
               .and(
                       choice(
                               string("28"),
                               string("6"),
                               Combinators.<Chr>eof().map(u -> "")
                       )
               ).map(a -> b -> a + b);

or (slightly cleaner):

final Parser<Chr, String> p =
        string("1")
                .andR(
                        choice(
                                string("28").map(u -> "128"),
                                string("6").map(u -> "16"),
                                Combinators.<Chr>eof().map(u -> "1")
                        )
                );

This now gives the desired result:

Success{value=128, next=StringInput{3,data="EOF"}
Success{value=16, next=StringInput{2,data="EOF"}
Success{value=1, next=StringInput{1,data="EOF"}

Hope that helps.

[matozoid]

Thanks for the help, I get it now.