XIOM Error Codes -- Reference

Each XIOM diagnostic carries a code. Codes are stable: once assigned they are never deleted or reassigned. The same pages are published on the documentation site: https://docs.xiom-lang.org/latest/error-codes/

Emitted codes

Code	Stage	Meaning	Doc
L001	Lexer	Source cannot be tokenized (bad literal, bad escape)	L001
P001	Parser	Grammar error, messages shaped expected X, found Y	P001
T001	Type checker	Type and semantic errors (umbrella code)	T001
E001	Borrow checker	Ownership / borrow rule violation	E001
C001	Codegen	Internal code generation failure	C001
W000	Checker	Non-fatal checker warning	W000
W001	Catalog	Module name collision	W001

Codes appear in CLI output as error[CODE]: line:col: message and in --diagnostics=json output as the code field of each diagnostic.

Reserved codes (not emitted)

The X family below was reserved in an early design and is not emitted by the current compiler. The numbers stay reserved so they can never be reused. The pages are kept for continuity: X0010 type mismatch, X0011 undefined variable, X0100 contract requires violation. Those situations are reported as T001 today, and a runtime contract trap prints Contract violation: requires: ... without a code.

Writing a new error code

1. Create docs/error_codes/{CODE}.md in the website repository
2. Include: short description, an example, fix, notes
3. Add the row and link to the emitted table above
4. Codes are never deleted -- mark deprecated codes as such

Using --explain

xiom --explain T001
# Prints docs/error_codes/T001.md when that directory is present in the
# current working directory, then a pointer to this index.

--explain resolves the file relative to the current directory, so it works inside a checkout that carries docs/error_codes/. Installed toolchains do not ship the reference yet; use the published pages above in that case.