Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 0 inputs and 3 outputs

What it means

reflectx.ToFunc0x3 adapts a Func to Func0x3 (0 inputs, 3 outputs) and panics unless the reflect type has exactly 0 inputs and 3 outputs. The panic message reports the actual function type with its input and output counts.

Solutions

Use ToFunc0x2 if the function now returns only two values.
Ensure the function is a plain closure with zero parameters and exactly three returns.
Verify with fn.Type().NumIn() and fn.Type().NumOut() before choosing the adapter, and pick the matching ToFuncNxx.

Example fix

// before
reflectx.ToFunc0x3(reflectx.MakeFunc(func() (int, int) { ... }))
// after
reflectx.ToFunc0x2(reflectx.MakeFunc(func() (int, int) { ... }))

Defensive patterns

Strategy: validation

Validate before calling

func isFunc0x3(fn reflectx.Func) bool {
    return fn.Type().NumIn() == 0 && fn.Type().NumOut() == 3
}

Type guard

func asFunc0x3(fn reflectx.Func) (reflectx.Func0x3, bool) {
    if f, ok := fn.(reflectx.Func0x3); ok { return f, true }
    return nil, false
}

Try / catch

func safeToFunc0x3(fn reflectx.Func) (f reflectx.Func0x3, err error) {
    defer func() {
        if r := recover(); r != nil { err = fmt.Errorf("ToFunc0x3: %v", r) }
    }()
    return reflectx.ToFunc0x3(fn), nil
}

Prevention

Ensure the closure has zero parameters and three returns.
Bound methods can add inputs — convert via a closure with an empty parameter list.
Confirm NumIn/NumOut before selecting the adapter variant.

When it happens

Trigger: Calling ToFunc0x3 or MakeFunc0x3 with a func whose signature is not func() (A, B, C) — e.g. func() (A, B) (two returns) or func(ctx, x) (A, B, C) (inputs present).

Common situations: Switching a generator from three returns to two (e.g. dropping the error) without changing the adapter; wrapping a function with receiver-bound parameters that appear as extra inputs in the reflect 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/0bd9ba869c088c29. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:188

}

func (c *shimFunc0x3) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc0x3) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc0x3) Call0x3() (any, any, any) {
	ret := c.inner.Call([]any{})
	_ = ret
	return ret[0], ret[1], ret[2]
}

func ToFunc0x3(c Func) Func0x3 {
	if c.Type().NumIn() != 0 || c.Type().NumOut() != 3 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 0 inputs and 3 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func0x3); ok {
		return sc
	}
	return &shimFunc0x3{inner: c}
}

func MakeFunc0x3(fn any) Func0x3 {
	return ToFunc0x3(MakeFunc(fn))
}

type Func0x4 interface {
	Func
	Call0x4() (any, any, any, any)
}

type shimFunc0x4 struct {
	inner Func

View on GitHub (pinned to 12126d8942)