Diagnostics and semantic errors

Learning objective

• Interpret diagnostics quickly and choose the right fix path.

Key syntax

Line 2, column 12: Incompatible return type: expected int but got string

Examples

• Common diagnostics:
• undeclared variable
• unknown function or class
• wrong argument count
• incompatible assignment or return type
• return outside function
• cyclic import or missing symbol

Frequent diagnostics and quick fixes

• Undeclared variable: 'x'
• Cause: variable not declared in current scope.
• Fix: declare with let/const or reference the correct symbol.

• Unknown function or class: 'Foo'
• Cause: missing import, typo, or wrong symbol name.
• Fix: add import and verify exact identifier.

• Incorrect number of arguments for 'fn': A != B
• Cause: call does not match function/method signature.
• Fix: update call arguments or signature.

• Cannot initialize 'x' of type list<T> with list<any>
• Cause: incompatible inferred list type.
• Fix: use a typed declaration (let x: list<T> = []) or compatible values.

• Cannot call member 'length' on type list<...>
• Cause: length() is not a Lucia list method.
• Fix: use len(listValue).

• Unexpected Token '=' near indexed write
• Cause in older compiler versions: indexed assignment target not enabled.
• Current behavior: obj[i] = value; is supported.

Common mistakes

• Applying quick fix without understanding root cause.
• Ignoring file/module boundaries when errors come from imports.

Debug workflow

• Read the first error first. Later diagnostics are often cascade effects.
• Verify imports and symbol names before touching complex logic.
• Reduce to a minimal reproducible snippet.
• Re-run after each fix to confirm the true root cause is solved.

Suggested practice

• Trigger 3 diagnostics intentionally and fix each with explanation.

Related

• declarations-let-const
• functions-and-builtins
• oop-and-modules