Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 6 inputs and 1 outputs

What it means

reflectx.ToFunc6x1 wraps a reflectx.Func into a Func6x1 (6 inputs, 1 output). It panics with this message when the func's reflect.Type has any other arity, because the shim returns ret[0] directly and would index out of range otherwise.

Solutions

Adjust the function to 6 inputs and exactly 1 return value.
Use ToFunc6x2 for a (T, error) style signature or ToFunc6x0 for void.
Cross-check the expected signature in the Beam registration and fix the mismatched side.

Example fix

// before
func fn(a A, b B, c C, d D, e E, f F) (T, error) { ... }
reflectx.ToFunc6x1(fn) // panics: 2 outputs

// after
func fn(a A, b B, c C, d D, e E, f F) T { ... }
reflectx.ToFunc6x1(fn)

Defensive patterns

Strategy: validation

Validate before calling

fn := reflect.TypeOf(fnAny)
if fn.Kind() == reflect.Func && fn.NumIn() == 6 && fn.NumOut() == 1 {
    _ = reflectx.ToFunc6x1(fnAny)
}

Type guard

func isFunc6x1(f interface{}) bool {
    t := reflect.TypeOf(f)
    return t != nil && t.Kind() == reflect.Func && t.NumIn() == 6 && t.NumOut() == 1
}

Try / catch

func safeToFunc6x1(c reflectx.Func) (f reflectx.Func6x1, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("ToFunc6x1 failed: %v", r)
        }
    }()
    return reflectx.ToFunc6x1(c), nil
}

Prevention

Treat (T, error) as 2 outputs; use the x2 helper or remove the error.
Verify parameter count after adding context or timestamp parameters.
Wrap-test all user functions in CI.
Match helper names to signatures during code review.

When it happens

Trigger: Calling reflectx.ToFunc6x1(c) where c.Type().NumIn() != 6 or c.Type().NumOut() != 1 — e.g. a 6-input function returning (T, error) or nothing.

Common situations: Map-style DoFns with 6 params returning 2 values; registration drift after adding a context/timestamp parameter; choosing the wrong ToFunc helper when converting to a fixed-arity type.

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/f71633d201becf19. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:1336

}

func (c *shimFunc6x1) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc6x1) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc6x1) Call6x1(arg0, arg1, arg2, arg3, arg4, arg5 any) any {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3, arg4, arg5})
	_ = ret
	return ret[0]
}

func ToFunc6x1(c Func) Func6x1 {
	if c.Type().NumIn() != 6 || c.Type().NumOut() != 1 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 6 inputs and 1 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func6x1); ok {
		return sc
	}
	return &shimFunc6x1{inner: c}
}

func MakeFunc6x1(fn any) Func6x1 {
	return ToFunc6x1(MakeFunc(fn))
}

type Func6x2 interface {
	Func
	Call6x2(any, any, any, any, any, any) (any, any)
}

type shimFunc6x2 struct {
	inner Func

View on GitHub (pinned to 12126d8942)