Code readthrough, comments

[bisqwit]

I had a brief read through your cons_expr.hpp. Fascinating code. Assorted comments below:

Category: Great idea!

— [[nodiscard]] attribute on ++(int), --(int), begin() etc.

Category: Could be better?

— pow(): Could use the much more efficient binary exponentiation algorithm: for(result = 1; power != 0; power >>= 1, base *= base) if(power & 1) result *= base; (similarly /= for negative exponents)

Category: Why the discrepancy?

— pow(): Parameter is long long, but is iterated using int.
— parse_float(): Accepts "-.4" but not ".4".
— parse_float(): Accepts "13.4e2" and "13.4e-2". Accepts "13e2" but not "13e-2".

Category: Errors?

— parse_float(): Accepts "-e2".
— next_token(): Interprets both "a\"b" and "a\\"b" as valid string constants.

[lefticus]

@bisqwit sorry I didn't take a closer look at this sooner. I've made a bunch of improvements to the number parser since then, as well. @leftibot review this issue. Does the current code already address these issues?

[leftibot]

I went through the current parse_number (formerly parse_float) and next_token against @bisqwit's list. Summary: most of the number-parser issues are fixed, but the string-escape bug in next_token is still there.

✅ Fixed in the current code

Verified via cons_expr_cli:

Input	Current result
.4	0.4 ✓
-.4	-0.4 ✓
13e2	1300 ✓
13e-2	0.13 ✓
-e2	rejected (falls through to identifier lookup, which fails) ✓

parse_number now lives at include/cons_expr/cons_expr.hpp:260 and handles all of these uniformly — the guard if (int_digits == 0 && frac_digits == 0) rejects -e2 while allowing .4/-.4, and the exponent branch accepts a leading - so 13e-2 parses.

The original freestanding pow() function (with long long parameter iterated via int, and the suggestion to use binary exponentiation) appears to have been removed entirely. The only remnant is the pow_10 lambda at cons_expr.hpp:272, used during float assembly. Two small observations about it:

• It's still for (int i = 0; i < power; ++i) while power is a std::integral auto — harmless for the float-parsing use case (exponents are tiny), but technically inconsistent. Changing int i to decltype(power) i (or std::remove_cvref_t<decltype(power)>) would match cleanly.
• Binary exponentiation is a nice-to-have here but the wins are negligible for powers under ~20. Not a bug.

[[nodiscard]] coverage on begin() etc. looks good; the ++(int) / --(int) operators referenced in the original comment don't appear to exist in the current code.

❌ Still broken: backslash handling in next_token

At cons_expr.hpp:357-369:

bool in_escape = false;
auto location = std::next(input.begin());
while (location != input.end()) {
  if (*location == chars::ch('\\')) {
    in_escape = true;          // <-- never toggles
  } else if (*location == chars::ch('"') && !in_escape) {
    ++location;
    break;
  } else {
    in_escape = false;
  }
  ++location;
}

Because \ unconditionally sets in_escape = true instead of toggling it, a literal escaped backslash (\\) leaves the tokenizer in escape mode, so the following " is eaten as if it were escaped.

Reproduced with a file containing just "a\\" (which should be a valid 1-character string):

Result: [Expected: "terminated string" got: (""a\\\n")]

And with "foo\\" "bar":

Result: [Expected: "id not found" got: (bar")]

— the tokenizer merged both string literals into one, leaving a stray bar".

The minimal fix is to toggle rather than set:

if (*location == chars::ch('\\')) {
  in_escape = !in_escape;
}

That way \\ returns to in_escape = false before the next character is examined, and the terminating " behaves correctly. process_string_escapes at cons_expr.hpp:661 already handles \\ properly on the second pass, so only the tokenizer needs the change. Worth adding regression tests for "a\\", "\\", and "foo\\" "bar" while we're in there.

[lefticus]

@leftibot add tests and fix for the escape issue.