Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 8 inputs and 0 outputs

What it means

reflectx.ToFunc8x0 adapts a Func as an 8-input/0-output (sink) function and panics if the wrapped signature differs. Any return values or a different input count trigger the panic because shimFunc8x0 performs a bare forwarding call. The message shows got vs want counts.

Solutions

Make the function take exactly 8 parameters and return nothing
If it returns values, use ToFunc8x1..ToFunc8xN as appropriate
Use the correct input-count converter if 8 is wrong (e.g. ToFunc7x0)
Catch or pre-check arity in dynamic registration paths

Example fix

// before
func(a, b, c, d, e, f2, g, h any) error { return nil } // 1 output
reflectx.ToFunc8x0(reflectx.NewFunc(fn)) // panics
// after
func(a, b, c, d, e, f2, g, h any) {} // 0 outputs
reflectx.ToFunc8x0(reflectx.NewFunc(fn)) // OK

Defensive patterns

Strategy: validation

Validate before calling

t := f.Type()
if t.NumIn() != 8 || t.NumOut() != 0 {
    return fmt.Errorf("ToFunc8x0 needs 8 in / 0 out, got %d in / %d out (%v)", t.NumIn(), t.NumOut(), t)
}

Type guard

func isFunc8x0(f reflectx.Func) bool {
    t := f.Type()
    return t.NumIn() == 8 && t.NumOut() == 0
}

Try / catch

func safeToFunc8x0(f reflectx.Func) (fn reflectx.Func8x0, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("conversion failed: %v", r)
        }
    }()
    fn = reflectx.ToFunc8x0(f)
    return
}

Prevention

Sinks return nothing; use Nx1+ converters if an error return is needed
Verify the 8-parameter signature after adding context/emitter args
Prefer MakeFunc8x0 with a typed function
Run a fast unit test that constructs the sink before launching pipelines

When it happens

Trigger: Calling ToFunc8x0 (or MakeFunc8x0) with a Func having inputs != 8 or any outputs (e.g. an 8-in/1-out function returning error).

Common situations: Sinks gaining an error return during refactors; an extra context/emitter parameter added or removed (7<->8 inputs) without swapping converters; reusing registration boilerplate from another stage.

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/d08955b41f9dce20. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:1705

}

func (c *shimFunc8x0) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc8x0) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc8x0) Call8x0(arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7 any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7})
	_ = ret
	return
}

func ToFunc8x0(c Func) Func8x0 {
	if c.Type().NumIn() != 8 || c.Type().NumOut() != 0 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 8 inputs and 0 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func8x0); ok {
		return sc
	}
	return &shimFunc8x0{inner: c}
}

func MakeFunc8x0(fn any) Func8x0 {
	return ToFunc8x0(MakeFunc(fn))
}

type Func8x1 interface {
	Func
	Call8x1(any, any, any, any, any, any, any, any) any
}

type shimFunc8x1 struct {
	inner Func

View on GitHub (pinned to 12126d8942)