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.

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