Error Model (mex normalization)

Ignition normalizes all runtime errors to MATLAB-style identifiers via a tiny helper mex(id, message). The VM constructs a canonical string "<IDENTIFIER>: <message>" and uses it consistently across all error sites. Tests assert on identifiers to prevent drift.

Namespacing

ERROR_NAMESPACE is currently "MATLAB" for all errors emitted by the VM. It is an arbitrary string defined in vm.rs and used to prefix all error identifiers, and left as "MATLAB" to ensure compatibility with existing post-script code that may exist in existing MATLAB codebases.

Standard identifiers and when they occur

• MATLAB:UndefinedFunction — unknown function or builtin at call
• MATLAB:UndefinedVariable — unknown variable (primarily a frontend HIR concern)
• MATLAB:NotEnoughInputs / MATLAB:TooManyInputs — function arity mismatch (user functions and some builtins)
• MATLAB:TooManyOutputs — requesting more outputs than a function defines
• MATLAB:VarargoutMismatch — requesting more varargout elements than available
• MATLAB:SliceNonTensor — attempted slicing on a non-tensor/non-string-array value
• MATLAB:IndexOutOfBounds — numeric index out of bounds
• MATLAB:SubscriptOutOfBounds — 2-D scalar subscript out of bounds
• MATLAB:IndexShape — logical mask shape mismatch
• MATLAB:IndexStepZero — range step was 0 in end arithmetic forms
• MATLAB:CellIndexType — unsupported cell index type
• MATLAB:CellSubscriptOutOfBounds — 2-D cell subscripting out of bounds
• MATLAB:ExpandError — illegal/unsupported argument expansion
• MATLAB:MissingSubsref / MATLAB:MissingSubsasgn — object lacks the overload
• MATLAB:ShapeMismatch — element-wise comparison or assignment shape mismatch
• MATLAB:CharError — failed to materialize CharArray

The VM also produces plain strings for some internal plumbing; they are wrapped by vm_bail! into the try/catch machinery so callers see normalized identifiers at the escape boundary.

Try/Catch, rethrow, and exception objects

• EnterTry(catch_pc, catch_var?) pushes a try frame; on failure, control jumps to catch_pc
• If catch_var is present, the VM binds the caught exception into the given variable as Value::MException
• PopTry pops the top try frame on successful completion of the try block
• rethrow with no argument rethrows the last caught exception (tracked thread-locally)

Internally, parse_exception splits "IDENT: message" using the last ": ", preserving nested identifiers.

Guidelines for contributors

• Prefer one canonical site per error condition and share it via helpers
• Include actionable details (function name, expected vs. got, etc.)
• Do not emit raw strings from VM instruction handlers; always normalize via mex
• Keep test coverage in sync with error identifiers to prevent regressions

See vm.rs for concrete mappings and tests under ignition and runtime crates (e.g., tests/control_flow.rs, tests/indexing_properties.rs).