how to write a compiler in 20 lines of code

gistfile1.perl

pmichaud@plum:~/rakudo$ cat x
grammar Expr {
    rule TOP { <expr> }
    rule expr { <integer> <addop> <integer> };
    token addop { '+' | '-' }
    token integer { \d+ }
}


class ExprActions {
    method TOP($/) { make $<expr>.ast }

    method expr($/) {
        my $a = $<integer>[0].ast;
        my $b = $<integer>[1].ast;
        make $<addop> eq '+' ?? $a + $b !! $a - $b;
    }

    method integer($/) { make +$/; }
}


my $match = Expr.parse("3+4", :actions(ExprActions));
say $match.ast;


pmichaud@plum:~/rakudo$ ./perl6 x
7
pmichaud@plum:~/rakudo$