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 7 inputs and 4 outputs

What it means

reflectx.ToFunc7x4 converts a Func into a 7-input/4-output typed function and panics on any arity mismatch. The shim returns ret[0]..ret[3], so exactly 4 outputs and 7 inputs are mandatory. This surfaces at graph-construction time as a panic with the offending func type in the message.

Source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:1664

}

func (c *shimFunc7x4) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc7x4) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc7x4) Call7x4(arg0, arg1, arg2, arg3, arg4, arg5, arg6 any) (any, any, any, any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3, arg4, arg5, arg6})
	_ = ret
	return ret[0], ret[1], ret[2], ret[3]
}

func ToFunc7x4(c Func) Func7x4 {
	if c.Type().NumIn() != 7 || c.Type().NumOut() != 4 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 7 inputs and 4 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func7x4); ok {
		return sc
	}
	return &shimFunc7x4{inner: c}
}

func MakeFunc7x4(fn any) Func7x4 {
	return ToFunc7x4(MakeFunc(fn))
}

type Func8x0 interface {
	Func
	Call8x0(any, any, any, any, any, any, any, any)
}

type shimFunc8x0 struct {
	inner Func

View on GitHub (pinned to 12126d8942)

Solutions

Adjust the function to exactly 7 parameters and 4 return values
Use the matching ToFuncNxM for the real signature (e.g. ToFunc7x3, ToFunc8x4)
Align the converter choice with the pipeline stage's expected outputs
Check NumIn/NumOut before converting in generic/plugin code

Example fix

// before
func(a, b, c, d, e, f2, g any) (any, any, any) { ... } // 3 outputs
reflectx.ToFunc7x4(reflectx.NewFunc(fn)) // panics
// after
func(a, b, c, d, e, f2, g any) (any, any, any, any) { ... } // 4 outputs
reflectx.ToFunc7x4(reflectx.NewFunc(fn)) // OK

Defensive patterns

Strategy: validation

Validate before calling

t := f.Type()
if t.NumIn() != 7 || t.NumOut() != 4 {
    return fmt.Errorf("ToFunc7x4 needs 7 in / 4 out, got %d in / %d out (%v)", t.NumIn(), t.NumOut(), t)
}

Type guard

func isFunc7x4(f reflectx.Func) bool {
    t := f.Type()
    return t.NumIn() == 7 && t.NumOut() == 4
}

Try / catch

func safeToFunc7x4(f reflectx.Func) (fn reflectx.Func7x4, err error) {
    defer func() {
        if r := recover(); r != nil {
            err = fmt.Errorf("conversion failed: %v", r)
        }
    }()
    fn = reflectx.ToFunc7x4(f)
    return
}

Prevention

Four outputs required — include every emitted collection plus error in the count
Update the converter alongside any 3->4 output refactor
Use typed MakeFunc7x4 for compile-time checks
Gate registrations behind construction-time smoke tests

When it happens

Trigger: Calling ToFunc7x4 (or MakeFunc7x4) with a Func whose signature has inputs != 7 or outputs != 4, e.g. a 7-in/3-out or 8-in/4-out function.

Common situations: Beam DoFns emitting multiple collections where one output was added (3->4) without updating the wrapper; arity drift after API refactors; mistakenly passing the raw Go func without the matching NewFunc/Func 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/e5e0083596655229. Report an issue: GitHub.