bool.grammar

token whitespace = /[ \n\t]+/ [ skip ];
token true = /true/;
token false = /false/;
token and = /and/;
token or = /or/;

rule S = expr;
rule expr = tkn | expr and tkn | expr or tkn;
rule tkn = true | false;

bool.sem

type boolean = $$ Bool $$

typerules boolean = [ S, expr, tkn ]

init = $$ $$

rule expr(0) = $$ $out = $0 $$
rule expr(1) = $$ $out = $0 & $2 $$
rule expr(2) = $$ $out = $0 | $2 $$
rule tkn(0) = $$ $out = true $$
rule tkn(1) = $$ $out = false $$

build.sh

#!/usr/bin/bash

pegasus < bool.grammar > bool.json
pegasus-crystalsem -l bool.json -a bool.sem -s bool.cr

crystal test.cr

test.cr

require "./bool.cr"

puts Pegasus::Generated.process("true or false or true").inspect