Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 4 inputs and 0 outputs

What it means

reflectx.ToFunc4x0 converts a generic Func to a strongly-typed Func4x0 (4 inputs, 0 outputs). It panics if the underlying reflect.Type does not have exactly NumIn()==4 and NumOut()==0. Beam throws this to guarantee the fixed-arity shim's invocation contract matches the wrapped function, catching the mismatch at conversion time with the actual type in the message.

Solutions

Make the function take exactly 4 parameters and return nothing.
If it returns a value or error, switch to the matching converter (ToFunc4x1, ToFunc4x2, ...).
Confirm the shape with reflect.TypeOf(fn).NumIn()/NumOut() before wrapping.
Move the error handling inside the function body (log/emit) if a no-output signature is required by the pipeline.

Example fix

// before
f := reflectx.ToFunc4x0(func(a int, b string, c bool, d float64) error { return nil }) // 1 output, panics
// after
f := reflectx.ToFunc4x0(func(a int, b string, c bool, d float64) {
    // handle a,b,c,d without returning
})

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn)
if t.NumIn() != 4 || t.NumOut() != 0 {
    panic(fmt.Sprintf("expected 4-in/0-out func, got %v (%d in, %d out)", t, t.NumIn(), t.NumOut()))
}
f := reflectx.ToFunc4x0(f)

Type guard

func isFunc4x0(fn any) bool {
    t := reflect.TypeOf(fn)
    return t != nil && t.Kind() == reflect.Func && t.NumIn() == 4 && t.NumOut() == 0
}

Try / catch

func safeToFunc4x0(f reflectx.Func) (out reflectx.Func4x0, err error) {
    defer func() { if r := recover(); r != nil { err = fmt.Errorf("ToFunc4x0: %v", r) } }()
    out = reflectx.ToFunc4x0(f)
    return
}

Prevention

Make consumer/sink functions truly return-free; log or emit errors internally.
Assert NumIn/NumOut in unit tests for every ToFunc4x0 target.
Change parameter count and converter call in the same commit.
Use closures for auxiliary state instead of extra parameters.

When it happens

Trigger: Calling reflectx.ToFunc4x0(f) where f.Type().NumIn() != 4 (e.g. 3 or 5 params) or f.Type().NumOut() != 0 (the function returns anything, including an error).

Common situations: Sink-style 4-argument functions that still return an error value, functions that gained or lost a context/emitter parameter during refactoring, or copy-pasted converter calls pointing at the wrong arity variant.

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/3b5a91cdcb1dbe49. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:885

}

func (c *shimFunc4x0) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc4x0) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc4x0) Call4x0(arg0, arg1, arg2, arg3 any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2, arg3})
	_ = ret
	return
}

func ToFunc4x0(c Func) Func4x0 {
	if c.Type().NumIn() != 4 || c.Type().NumOut() != 0 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 4 inputs and 0 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func4x0); ok {
		return sc
	}
	return &shimFunc4x0{inner: c}
}

func MakeFunc4x0(fn any) Func4x0 {
	return ToFunc4x0(MakeFunc(fn))
}

type Func4x1 interface {
	Func
	Call4x1(any, any, any, any) any
}

type shimFunc4x1 struct {
	inner Func

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