Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 5 inputs and 3 outputs

What it means

reflectx.ToFunc5x3 wraps a reflectx.Func into a Func5x3 (5 inputs, 3 outputs). It panics with this message when the func's reflect.Type has any arity other than 5 inputs and 3 outputs. The shim needs the exact arity to return ret[0], ret[1], ret[2].

Solutions

Align the function to exactly 5 inputs and 3 return values.
Use the ToFunc variant matching the real output count (ToFunc5x2, ToFunc5x4, etc.).
Audit the ProcessElement signature against the fixed-arity type used at registration time.

Example fix

// before
func fn(a A, b B, c C, d D, e E) (T, U) { ... }
reflectx.ToFunc5x3(fn) // panics: 2 outputs

// after
func fn(a A, b B, c C, d D, e E) (T, U, V) { ... }
reflectx.ToFunc5x3(fn)

Defensive patterns

Strategy: validation

Validate before calling

fn := reflect.TypeOf(fnAny)
if fn.Kind() == reflect.Func && fn.NumIn() == 5 && fn.NumOut() == 3 {
    _ = reflectx.ToFunc5x3(fnAny)
}

Type guard

func isFunc5x3(f interface{}) bool {
    t := reflect.TypeOf(f)
    return t != nil && t.Kind() == reflect.Func && t.NumIn() == 5 && t.NumOut() == 3
}

Try / catch

func safeToFunc5x3(c reflectx.Func) (f reflectx.Func5x3, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("ToFunc5x3 failed: %v", r)
        }
    }()
    return reflectx.ToFunc5x3(c), nil
}

Prevention

For multi-output functions, verify the emitted-value count matches the registered arity exactly.
Prefer generic registration APIs over fixed-arity helpers when signatures are in flux.
Add reflection-based assertions near DoFn definitions.
Review signature diffs in PRs for input/output count changes.

When it happens

Trigger: Calling reflectx.ToFunc5x3(c) where c.Type().NumIn() != 5 or c.Type().NumOut() != 3 — e.g. a function emitting 2 or 4 values, or taking 4 or 6 inputs.

Common situations: DoFns emitting multiple outputs where the emitted-value count was changed (added/dropped a return) without updating the registration; mixing up (value..., error) conventions with pure multi-emission signatures.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/33acd3a99c23ee05. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:1213

}

func (c *shimFunc5x3) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc5x3) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc5x3) Call5x3(arg0, arg1, arg2, arg3, arg4 any) (any, any, any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3, arg4})
	_ = ret
	return ret[0], ret[1], ret[2]
}

func ToFunc5x3(c Func) Func5x3 {
	if c.Type().NumIn() != 5 || c.Type().NumOut() != 3 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 5 inputs and 3 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func5x3); ok {
		return sc
	}
	return &shimFunc5x3{inner: c}
}

func MakeFunc5x3(fn any) Func5x3 {
	return ToFunc5x3(MakeFunc(fn))
}

type Func5x4 interface {
	Func
	Call5x4(any, any, any, any, any) (any, any, any, any)
}

type shimFunc5x4 struct {
	inner Func

View on GitHub (pinned to 12126d8942)