Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 0 outputs

What it means

ToFunc2x0 adapts a generic reflectx Func into a Func2x0 (2 inputs, 0 outputs, a binary consumer). It panics when the wrapped function's type does not have exactly 2 inputs and 0 outputs, so arity mismatches fail at construction rather than at call time.

Solutions

Make the wrapped function take exactly 2 inputs and return nothing.
Use the ToFunc/MakeFuncNxM variant matching the actual signature.
Verify c.Type().NumIn()==2 && c.Type().NumOut()==0 before wrapping.

Example fix

// before
reflectx.MakeFunc2x0(func(a, b int) int { return a + b }) // panics: 1 output
// after
reflectx.MakeFunc2x0(func(a, b int) { _ = a + b })

Defensive patterns

Strategy: validation

Validate before calling

t := c.Type()
if t.NumIn() != 2 || t.NumOut() != 0 {
    return fmt.Errorf("func %v has %d ins/%d outs; ToFunc2x0 needs 2 ins/0 outs", t, t.NumIn(), t.NumOut())
}

Type guard

func isFunc2x0(c reflectx.Func) bool {
    return c.Type().NumIn() == 2 && c.Type().NumOut() == 0
}

Try / catch

func safeToFunc2x0(c reflectx.Func) (f reflectx.Func2x0) {
    defer func() {
        if r := recover(); r != nil {
            log.Fatalf("ToFunc2x0 arity mismatch: %v", r)
        }
    }()
    return reflectx.ToFunc2x0(c)
}

Prevention

Sinks must return nothing; strip error returns or switch to a variant with outputs.
Check parameter count when converting a 1-input consumer to a 2-input one.
Assert arity in tests for all sink wrappers.

When it happens

Trigger: Calling reflectx.ToFunc2x0 (or its MakeFunc2x0 helper) with a Func whose Type().NumIn() != 2 or Type().NumOut() != 0, e.g. a one-argument sink or one that returns a value.

Common situations: Writing a combine/sink function that lost or gained a parameter; wrapping a function that still returns an error; using the wrong helper variant after a signature change.

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/03868318aab0ce8d. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:475

}

func (c *shimFunc2x0) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc2x0) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc2x0) Call2x0(arg0, arg1 any) {
	ret := c.inner.Call([]any{arg0, arg1})
	_ = ret
	return
}

func ToFunc2x0(c Func) Func2x0 {
	if c.Type().NumIn() != 2 || c.Type().NumOut() != 0 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 0 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func2x0); ok {
		return sc
	}
	return &shimFunc2x0{inner: c}
}

func MakeFunc2x0(fn any) Func2x0 {
	return ToFunc2x0(MakeFunc(fn))
}

type Func2x1 interface {
	Func
	Call2x1(any, any) any
}

type shimFunc2x1 struct {
	inner Func

View on GitHub (pinned to 12126d8942)