Incompatible func type: got func %v with %v inputs and %v ou

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 6 inputs and 4 outputs

What it means

reflectx.ToFunc6x4 wraps an arbitrary reflectx Func as a strongly-typed 6-input/4-output function. It panics when the wrapped function's reflect.Type does not have exactly 6 inputs and 4 outputs, because the generated shim cannot adapt an arity mismatch. This library uses panics for programmer errors detected at pipeline-graph construction time.

Source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:1459

}

func (c *shimFunc6x4) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc6x4) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc6x4) Call6x4(arg0, arg1, arg2, arg3, arg4, arg5 any) (any, any, any, any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3, arg4, arg5})
	_ = ret
	return ret[0], ret[1], ret[2], ret[3]
}

func ToFunc6x4(c Func) Func6x4 {
	if c.Type().NumIn() != 6 || c.Type().NumOut() != 4 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 6 inputs and 4 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func6x4); ok {
		return sc
	}
	return &shimFunc6x4{inner: c}
}

func MakeFunc6x4(fn any) Func6x4 {
	return ToFunc6x4(MakeFunc(fn))
}

type Func7x0 interface {
	Func
	Call7x0(any, any, any, any, any, any, any)
}

type shimFunc7x0 struct {
	inner Func

View on GitHub (pinned to 12126d8942)

Solutions

Count the inputs and outputs of the function passed in and fix it to have exactly 6 parameters and 4 return values
If the function intentionally has a different arity, call the matching ToFuncNxM converter (e.g. ToFunc7x4) instead
If it should return fewer/more values, use a different FuncNxM target type or change the call site of the conversion
Recover from the panic in tests to surface the message, since it prints got/want arity and the func type

Example fix

// before
f := reflectx.NewFunc(func(a, b, c, d, e any) (any, any, any) { ... })
fn := reflectx.ToFunc6x4(f) // panics: 5 inputs, 3 outputs
// after
f := reflectx.NewFunc(func(a, b, c, d, e, f2 any) (any, any, any, any) { ... })
fn := reflectx.ToFunc6x4(f)

Defensive patterns

Strategy: validation

Validate before calling

t := f.Type()
if t.NumIn() != 6 || t.NumOut() != 4 {
    return fmt.Errorf("ToFunc6x4 needs 6 in / 4 out, got %d in / %d out (%v)", t.NumIn(), t.NumOut(), t)
}

Type guard

func isFunc6x4(f reflectx.Func) bool {
    t := f.Type()
    return t.NumIn() == 6 && t.NumOut() == 4
}

Try / catch

func safeToFunc6x4(f reflectx.Func) (fn reflectx.Func6x4, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("conversion failed: %v", r)
        }
    }()
    fn = reflectx.ToFunc6x4(f)
    return
}

Prevention

Match converter choice to signature: NxM must equal the Go function's parameter and return counts
Re-run arity checks after every signature refactor
Prefer typed helpers (MakeFunc6x4 with a concrete Go func) so the compiler catches mismatches
Cover converter calls in unit tests so panics surface early

When it happens

Trigger: Calling ToFunc6x4 (directly or via MakeFunc6x4) with a Func whose underlying Go signature has any number of inputs other than 6 or any number of outputs other than 4, e.g. wrapping a func(a,b,c,d,e any) (x,y,z any) (5 in, 3 out).

Common situations: DoFn methods or combinators registered with the wrong arity for the emitter/consumer being built; code changed from returning 3 values to 4 (e.g. adding an error) without updating the conversion; copy-paste of a ToFunc5x4-style registration where one parameter was forgotten; generics/type-parameter refactors changing the signature silently.

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/4012306d914ddcf1. Report an issue: GitHub.