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This chapter describes errors for data languages. An error checklist is often a nice, concise way to describe a language.

Related: Oils Error Catalog, With Hints describes errors in code.

(in progress)

UTF8

J8 Notation is built on UTF-8, so let's summarize UTF-8 errors.

err-utf8-encode

Oils stores strings as UTF-8 in memory, so it doesn't encode UTF-8 often.

But it may have a function to encode UTF-8 from a List[Int]. These errors would be handled:

1. Integer greater than max code point
2. Code point in the surrogate range

err-utf8-decode

A UTF-8 decoder should handle these errors:

1. Overlong encoding. In UTF-8, each code point should be represented with the fewest possible bytes.
   • Overlong encodings are the equivalent of writing the integer 42 as 042, 0042, 00042, etc. This is not allowed.
2. Surrogate code point. The sequence decodes to a code point in the surrogate range, which is used only for the UTF-16 encoding, not for string data.
3. Exceeds max code point. The sequence decodes to an integer that's larger than the maximum code point.
4. Bad encoding. A byte is not encoded like a UTF-8 start byte or a continuation byte.
5. Incomplete sequence. Too few continuation bytes appeared after the start byte.

J8 String

J8 strings extend JSON strings, and are a primary building block of J8 Notation.

err-j8-str-encode

J8 strings can represent any string — bytes or unicode — so there are no encoding errors.

err-j8-str-decode

1. Escape sequence like \u{dc00} should not be in the surrogate range.
   • This means it doesn't represent a real character. Byte escapes like \yff should be used instead.
2. Escape sequence like \u{110000} is greater than the maximimum Unicode code point.
3. Byte escapes like \yff should not be in u'' string.
   • By design, they're only valid in b'' strings.

J8 Lines

Roughly speaking, J8 Lines are an encoding for a stream of J8 strings. In YSH, it's used by @(split command sub).

err-j8-lines-encode

Like J8 strings, J8 Lines have no encoding errors by design.

err-j8-lines-decode

1. Any error in a J8 quoted string.
   • e.g. no closing quote, invalid UTF-8, invalid backslash escape, ...
2. A line with a quoted string has extra text after it.
   • e.g. "mystr" extra.
3. An unquoted line is not valid UTF-8.

JSON

err-json-encode

JSON encoding has these errors:

1. Object of this type can't be serialized.
   • For example, Str List Dict are Oils objects can be serialized, but Eggex Func Range can't.
2. Circular reference.
   • e.g. a Dict that points to itself, a List that points to itself, and other permutations
3. Float values of NaN, Inf, and -Inf can't be encoded.
   • TODO: option to use null like JavaScript.

Note that invalid UTF-8 bytes like 0xfe produce a Unicode replacement character, not a hard error.

err-json-decode

1. The encoded message itself is not valid UTF-8.
   • (Typically, you need to check the unescaped bytes in string literals "abc\n").
2. Lexical error, like
   • the message +
   • an invalid escape "\z" or a truncated escape "\u1"
   • A single quoted string like u''
3. Grammatical error
   • like the message }{
4. Unexpected trailing input
   • like the message 42] or {}]

JSON8

err-json8-encode

JSON8 has the same encoding errors as JSON.

However, the encoding is lossless by design. Instead of invalid UTF-8 being turned into a Unicode replacment character, it can use J8 strings with byte escapes like b'byte \yfe\yff'.

err-json8-decode

JSON8 has the same decoding errors as JSON, plus J8 string decoding errors.

See err-j8-str-decode.