facebook/flow · error

Records cannot have setters

Error message

Records cannot have setters

What it means

The setter counterpart of the record restrictions: MethodKind::Set in a record body panics in the typing pass. Records are immutable data carriers in Flow's design, and a setter would imply mutable state, so the typer aborts rather than type it.

Source

Thrown at rust_port/crates/flow_typing_statement/src/statement.rs:18863

                                            kind: kind_c,
                                            static_: static_c,
                                            override_: false,
                                            ts_accessibility: None,
                                            decorators: vec![].into(),
                                            comments: method_comments_c,
                                        },
                                    ))
                                };

                                match kind {
                                    ast::class::MethodKind::Constructor => {
                                        panic!("Records cannot have constructors")
                                    }
                                    ast::class::MethodKind::Get => {
                                        panic!("Records cannot have getters")
                                    }
                                    ast::class::MethodKind::Set => {
                                        panic!("Records cannot have setters")
                                    }
                                    ast::class::MethodKind::Method => {
                                        check_duplicate_name(
                                            &mut public_seen_names,
                                            method_id_loc.dupe(),
                                            &Name::new(name.dupe()),
                                            static_,
                                            false,
                                            ClassMemberKind::ClassMemberMethod,
                                        );
                                        class_sig::add_method(
                                            static_,
                                            false,
                                            false,
                                            Name::new(name),
                                            method_id_loc,
                                            Some(func_loc),
                                            static_

View on GitHub (pinned to 5c86586199)

Solutions

1. Remove the setter from the record body.
2. Model updates functionally: function setN(c: Counter, v: number): Counter { return { ...c, n: v }; }.
3. Use a class if in-place mutation is genuinely required.
4. Reject setter members in record bodies via a lint/codemod check.

Example fix

// before
record Counter {
  n: number;
  set n(v: number) { this.n = v; }
}

// after: return a new record instead of mutating
record Counter { n: number; }
function setN(c: Counter, v: number): Counter { return { ...c, n: v }; }

// lint before typechecking: records must not contain setters
function recordHasSetter(node) {
  return node.body.some((el) => el.type === 'Method' && el.kind === 'set');
}
if (recordHasSetter(recordNode)) reportError(recordNode, 'records cannot have setters');

function recordMemberAllowed(el) {
  if (el.type === 'Property') return true;
  if (el.type !== 'Method') return false;
  return el.kind === 'method' && el.key.type === 'Identifier';
}

When it happens

Trigger: Typechecking a record declaration containing a setter — record Counter { n: number; set n(v: number) { ... } } — once the parser has admitted the member.

Common situations: Porting mutable classes to records without removing mutation; IDE refactorings generating setters; codemods that copy accessor pairs into record bodies.

Related errors

• Records cannot have constructors
• Records cannot have getters
• Records cannot have private methods
• Records can only have identifier method keys
• Unexpected function parameter ${param.type}

AI-assisted analysis of facebook/flow@5c86586199 (2026-08-20). Data as JSON: /api/errors/4b670b65e235f11b. Report an issue: GitHub.