Feature: compile-time conditionals and iterations

[epoberezkin]

{{#?}} and {{#~}}

[strrchr]

The following is just a try (can copy and paste it to node REPL) for demonstration that how to implement compile-time conditionals:

let src3 = `
Hello!
{{& def.DEBUG }}
the debug version...
{{&& def.DEBUG2 }}
the debug2 version...
{{&&}}
the release version...
aaa
{::{& def.HAS_BBB }::}
bbb
{::{&}::}
ccc
{{&}}
`;

let def = {
  HAS_BBB: true,
};

function resolve_static_if(s) {
  return s.replace(/\{(:*)\{&\s*([\s\S]*?)\s*\}\1\}([\s\S]*?)\{\1\{&\s*\}\1\}/g, function (m, mark, if_cond, block) {
    console.log("GOT", if_cond);
    for (;;) {
      assert(mark !== undefined);
      assert(if_cond !== undefined);
      assert(block !== undefined);
      if_cond = if_cond === "" ? true : new Function("def", "return " + if_cond)(def);
      let p = new RegExp("^([\\s\\S]*?)(?:\\{"+mark+"\\{&&\\s*([\\s\\S]*?)\\s*\\}"+mark+"\\}([\\s\\S]*?))?$");
      console.log("PATTERN", p);
      console.log("BLOCK", block.length);
      m = p.exec(block);
      //console.log("m:", m);
      let if_part = m[1];
      if (if_cond) return resolve_static_if(if_part);
      if_cond = m[2];
      if (if_cond === undefined) return "";
      block = m[3];
    }
  });
}

let dst3 = resolve_static_if(src3);
console.log("----\n"+dst3+"\n----");

Features:

• Just using recursion and replace(), no lexer and parser
• Using {:::{& cond }:::} syntax to distinguish nesting, because no lexer and parser
• Support elseif, using syntax {{&& cond }}
• Empty condition means true

I think the core functionality can be used in resolveDefs() of doT, after resolving c.define, before resolving c.use.