Errors

Vesper reports errors from several stages of execution.

The implementation uses typed errors derived from VesperError:

Error type	Stage
CliError	command-line arguments and file I/O
LexerError	source tokenization
ParserError	AST construction
ModuleError	module resolution, loading, and cycles
ResolverError	name and scope resolution
TypeError	static type checking
RuntimeError	interpretation and evaluation

The CLI catches these errors at the pipeline boundary and reports the phase, source file, and any line/column information already available in the diagnostic.

Lexical errors

These occur when the lexer cannot recognize source text.

Examples include malformed literals or invalid characters.

Parse errors

These occur when source text does not match Vesper’s grammar.

Example:

let x = ;

Type errors

These occur when expressions or statements violate the static type system.

Example:

let x: int = "hello";

Mutability errors

Attempting to assign to an immutable binding is invalid.

let x = 10;
x = 20;

Function-call errors

The checker/interpreter can reject invalid function calls, including incompatible argument types, missing required arguments, or too many arguments.

Error locations

Vesper tracks source line and column information for tokens and statements. This allows diagnostics to identify where an error occurred.

As the compiler matures, error messages should consistently follow:

[PHASE] file:line:column: error: message

For pipeline diagnostics, run the interpreter with --verbose or -V.

The interpreter uses strict parsing: a parser error stops execution before resolution, type checking, or interpretation. The formatter uses recovery parsing for recoverable delimiter errors so it can normalize source such as a missing semicolon. Errors are color-coded by phase when printed in a terminal.