Incompatible func type: got func

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 1 inputs and 3 outputs

What it means

ToFunc1x3 adapts a generic reflectx Func into a Func1x3 (1 input, 3 outputs). It panics when the wrapped function's reflect type does not have exactly 1 input and 3 outputs, failing fast at adaptation time.

Solutions

Change the function to return exactly 3 values with 1 input.
Use the ToFunc/MakeFuncNxM variant matching the actual arity.
Check NumIn()/NumOut() on the Func type first.

Example fix

// before
reflectx.MakeFunc1x3(func(x int) (int, int) { return x, x }) // panics: 2 outputs
// after
reflectx.MakeFunc1x3(func(x int) (int, int, int) { return x, x, x })

Defensive patterns

Strategy: validation

Validate before calling

t := c.Type()
if t.NumIn() != 1 || t.NumOut() != 3 {
    return fmt.Errorf("func %v has %d ins/%d outs; ToFunc1x3 needs 1 in/3 outs", t, t.NumIn(), t.NumOut())
}

Type guard

func isFunc1x3(c reflectx.Func) bool {
    return c.Type().NumIn() == 1 && c.Type().NumOut() == 3
}

Try / catch

func safeToFunc1x3(c reflectx.Func) (f reflectx.Func1x3) {
    defer func() {
        if r := recover(); r != nil {
            log.Fatalf("ToFunc1x3 arity mismatch: %v", r)
        }
    }()
    return reflectx.ToFunc1x3(c)
}

Prevention

Count return values explicitly when writing multi-output functions.
Keep helper variant selection (MakeFunc1x3) reviewed together with the function literal.
Write a smoke test that constructs every wrapper at startup.

When it happens

Trigger: Calling reflectx.ToFunc1x3 or MakeFunc1x3 with a Func whose Type().NumIn() != 1 or Type().NumOut() != 3.

Common situations: A multi-output function updated to return a different number of values without updating the adapter call; copying a 1x2 adapter and forgetting to update the expected output count.

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/ea657bad393f445d. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:393

}

func (c *shimFunc1x3) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc1x3) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc1x3) Call1x3(arg0 any) (any, any, any) {
	ret := c.inner.Call([]any{arg0})
	_ = ret
	return ret[0], ret[1], ret[2]
}

func ToFunc1x3(c Func) Func1x3 {
	if c.Type().NumIn() != 1 || c.Type().NumOut() != 3 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 1 inputs and 3 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func1x3); ok {
		return sc
	}
	return &shimFunc1x3{inner: c}
}

func MakeFunc1x3(fn any) Func1x3 {
	return ToFunc1x3(MakeFunc(fn))
}

type Func1x4 interface {
	Func
	Call1x4(any) (any, any, any, any)
}

type shimFunc1x4 struct {
	inner Func

View on GitHub (pinned to 12126d8942)