Options, results and JSON

Expected failures are values. Opt[T] is some(x) or none, and Res[T, E] is ok(x) or err(e). match takes them apart, and these methods cover the common cases:

o.and_then(f) and r.and_then(f) chain a function that gives an Opt, or a Res with the same error type: s.to_int.and_then(half).

JSON

json.encode(v) gives a Str, and json.decode[T](s) gives Res[T, Str]. Records are objects in field order, variants without fields are "Name" and with fields {"tag": "Name", ...}, Opt is the value or null (a missing field decodes as none), Map[Str, V] is an object and other maps are [[k, v], ...], lists, sets, heaps and tuples are arrays, newtypes are their inner value, and non-finite floats encode as null. HashMap and HashSet encode like Map and Set (in key order), Time as an ISO 8601 string, Duration as milliseconds, BigInt as a JSON integer of any length, Dec as a string such as "12.50" (a number also decodes), and Bits as a string of 0 and 1 with bit 0 last. Regex has no JSON form. Ratio and Locale encode as their fields ({"num": -3, "den": 4}, {"tag": "ja-JP"}) but do not decode (E_JSON), since decoding could not check normalization or the tag; decode the parts and call ratio_big or locale. Decode errors name the path: lines[0].qty: expected Int, found a string; malformed text gives invalid JSON. Types with functions or secrets are rejected with E_JSON; where clauses are checked after decoding, and one that fails is an err naming the field, title: must satisfy _.len > 0. A tuple target is written in parentheses, json.decode[(Int, Str)](s). A generic function may decode its type parameter, fn dec[T](s: Str) -> Res[T, Str] = json.decode[T](s): it is compiled once per type it is called with, and each call is checked to be decodable (dec[Locale] is E_JSON). json.encode(x) with x: T needs [T: Json].