notes

Syntax error recovery

Background

Whitespace syntax is not self-synchronizing, so syntax errors in the middle of a Whitespace document change the meaning of the following instructions. Programs often have missing or or extra line feeds at the end when copied from webpages, but this only affects the final instruction. When editing a document, however, insertions and deletions outside of numbers, labels, and comments will invalidate later instructions. To improve the editing experience, error-recovery heuristics are needed to invalidate the smallest ranges possible.

Long lines

A long line is usually a control flow instruction with a UTF-8 label or has a large push at the end.

To detect a labeled instruction:

1. Decode the line as UTF-8 (S is 0, T is 1) in reverse from the right in groups of 8, until less than 8 characters remain. This is the label.
2. Check the start of the line before the label:
   • If two characters remain, consume L from the previous line and emit the matched instruction:
     • LSS …L: label
     • LST …L: call
     • LTS …L: jz
     • LTT …L: jn
   • If no characters remain and the line is preceded with LSL, then consume LSL and emit jmp:
     • LSL …L: jmp
3. If a UTF-8 encoding error is encountered, then try to interpret it instead as an integer label in the range [0, 256) without leading zeros:
   • If the line starts with S(S|T)T and is 10 characters long or less, consume L from the previous line and emit the matched instruction:
     • LSS T…L: label
     • LST T…L: call
   • If the line starts with T, is 8 characters long or less, and is preceded by LSL, then consume LSL and emit jmp:
     • LSL T…L: jmp
   • If the line starts with T(S|T)T and is 10 characters long or less, consume L from the previous line and emit the matched instruction:
     • LTS T…L: jz
     • LTT T…L: jn

Otherwise, the line likely ends with a push. There may be zero or more add, sub, div, mod, store, or retrieve instructions before the push. Since the start of the push cannot be determined by scanning backwards, skip this line and resume parsing on the next line.

Disconnected labels

Collect all labels defined up until the syntax error and attempt to resume parsing at any position that would be a valid call, jmp, jz, or jn to that label.

Collect all call and jump targets for which a label has not been defined up until the syntax error and attempt to resume parsing at any position that would be a valid label.

Undefined instructions

Since the Whitespace instruction prefix tree is not full, there are some sequences that an instruction can not start with: STT, TSTL, TSL, TTL, TLSL, TLTL, TLL, LLS, and LLT. Assume then, that any occurrences of these sequences are valid and occur elsewhere in an instruction. Those contain L can not occur within numbers or labels, so some combinations are unambiguous.

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