Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 1 outputs

What it means

ToFunc2x1 adapts a generic reflectx Func into a Func2x1 (2 inputs, 1 output, a binary merge function). It panics unless the wrapped function has exactly 2 inputs and 1 output, catching signature mismatches at adaptation time.

Solutions

Make the function take exactly 2 inputs and return exactly 1 value.
Pick the matching arity variant.
Check NumIn()/NumOut() on the Func's type first.

Example fix

// before
reflectx.MakeFunc2x1(func(a, b int) (int, error) { return a + b, nil }) // panics
// after
reflectx.MakeFunc2x1(func(a, b int) int { return a + b })

Defensive patterns

Strategy: validation

Validate before calling

t := c.Type()
if t.NumIn() != 2 || t.NumOut() != 1 {
    return fmt.Errorf("func %v has %d ins/%d outs; ToFunc2x1 needs 2 ins/1 out", t, t.NumIn(), t.NumOut())
}

Type guard

func isFunc2x1(c reflectx.Func) bool {
    return c.Type().NumIn() == 2 && c.Type().NumOut() == 1
}

Try / catch

func safeToFunc2x1(c reflectx.Func) (f reflectx.Func2x1) {
    defer func() {
        if r := recover(); r != nil {
            log.Fatalf("ToFunc2x1 arity mismatch: %v", r)
        }
    }()
    return reflectx.ToFunc2x1(c)
}

Prevention

Merge/combine functions should not return errors in this API; handle errors outside the wrapper.
Verify both inputs exist when adapting from 1x1 to 2x1.
Keep a test that builds all combiners at init.

When it happens

Trigger: Calling reflectx.ToFunc2x1 with a Func whose Type().NumIn() != 2 or Type().NumOut() != 1, e.g. a combiner that also returns an error or takes a context.

Common situations: Merge/combine functions refactored to return (V, error); passing a 1-input function to a 2-input helper; wrong MakeFuncNxM variant chosen.

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/9358d96d752eeffc. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:516

}

func (c *shimFunc2x1) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc2x1) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc2x1) Call2x1(arg0, arg1 any) any {
	ret := c.inner.Call([]any{arg0, arg1})
	_ = ret
	return ret[0]
}

func ToFunc2x1(c Func) Func2x1 {
	if c.Type().NumIn() != 2 || c.Type().NumOut() != 1 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 2 inputs and 1 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func2x1); ok {
		return sc
	}
	return &shimFunc2x1{inner: c}
}

func MakeFunc2x1(fn any) Func2x1 {
	return ToFunc2x1(MakeFunc(fn))
}

type Func2x2 interface {
	Func
	Call2x2(any, any) (any, any)
}

type shimFunc2x2 struct {
	inner Func

View on GitHub (pinned to 12126d8942)