parse: \char typesets a character by code, instead of dropping the formula - #9
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…rmula \char is a TeX primitive that typesets the character at a code point. It reaches the math layer mostly through macro expansion — \char`\^ for a literal caret is the commonest form — so an unknown \char dropped the *whole* enclosing formula. In the 200-paper arXiv census it is the top structural math-drop cause by document count: 11 papers, 118 occurrences. \char now parses the TeX number forms that reach math — `<char> and `\<sym> (the code point of that one character; the tokenizer turns a control symbol like \^ into a tCtrl whose text is the character), "<hex>, and a decimal run — and emits the glyph at that code point. Unparseable input is a normal error, as before. Verified: the corpus form \char`\^ and the hex/decimal/backtick-char forms all render in both inline and display styles; full suite, go vet and gofmt clean. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The first test exercised the happy paths; add the rejecting paths (no number, non-digit, a lone or dangling backtick, a superscript after the backtick, an empty control name, above-U+10FFFF and int32-overflow codes) and a hex form with letters, so parseCharCode and isBaseDigit are fully covered and the coverage gate holds. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This was referenced Sep 1, 2026
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What
\charis a TeX primitive that typesets the character at a code point. It reaches the math layer mostly through macro expansion —\char`\^for a literal caret is the commonest form — so an unknown\chardropped the whole enclosing formula. In the 200-paper arXiv census it is the top structural math-drop cause by document count: 11 papers, 118 occurrences (setting aside per-document user macros).How
\charnow parses the TeX number forms that reach math and emits the glyph at that code point:`<char>and`\<sym>— the code point of that one character (the tokenizer turns a control symbol like\^into atCtrlwhose text is the character);"<hex>— hexadecimal;Unparseable input remains a normal error, exactly as before.
Verification
\char`\^plus the backtick-char, hex and decimal forms all render in both inline and display styles.go-tex/mathsuite,go vet,gofmtall clean.Part of a worst-first math-drop robustness pass (biblio: TeX82
\char). A follow-up will bump the engine'sgo-tex/mathdependency so the 11 papers recover end-to-end.🤖 Generated with Claude Code