go-delve/delve · error

can not compare %s variables

Error message

can not compare %s variables

What it means

For equality comparisons of composite kinds — slice, map, func and chan — Delve refuses to compare variables since Go itself only allows comparing those to nil. Because == on these kinds cannot be evaluated structurally, compareOp returns this error instead of guessing.

Source

Thrown at pkg/proc/eval.go:2676

	switch xv.Kind {
	case reflect.Ptr:
		eql = xv.Children[0].Addr == yv.Children[0].Addr
	case reflect.Array:
		if int64(len(xv.Children)) != xv.Len || int64(len(yv.Children)) != yv.Len {
			return false, errors.New("array too long for comparison")
		}
		eql, err = equalChildren(xv, yv, true)
	case reflect.Struct:
		if len(xv.Children) != len(yv.Children) {
			return false, nil
		}
		if int64(len(xv.Children)) != xv.Len || int64(len(yv.Children)) != yv.Len {
			return false, errors.New("structure too deep for comparison")
		}
		eql, err = equalChildren(xv, yv, false)
	case reflect.Slice, reflect.Map, reflect.Func, reflect.Chan:
		return false, fmt.Errorf("can not compare %s variables", xv.Kind.String())
	case reflect.Interface:
		if xv.Children[0].RealType.String() != yv.Children[0].RealType.String() {
			eql = false
		} else {
			eql, err = compareOp(token.EQL, &xv.Children[0], &yv.Children[0])
		}
	default:
		return false, fmt.Errorf("unimplemented comparison of %s variables", xv.Kind.String())
	}

	if op == token.NEQ {
		return !eql, err
	}
	return eql, err
}

func (v *Variable) isNil() bool {
	switch v.Kind {

View on GitHub (pinned to a23773e6c3)

Solutions

1. Compare element-wise: `p s1[0] == s2[0] && s1[1] == s2[1]`.
2. Compare lengths then elements: `p len(s1) == len(s2)` before element checks.
3. Compare maps by iterating keys, or compare a hash/serialized form if available.

Example fix

// before
p s1 == s2
// after
p len(s1) == len(s2) && s1[0] == s2[0]

// slices/maps/funcs/chans are only comparable to nil in Go
switch v.Kind {
case reflect.Slice, reflect.Map, reflect.Func, reflect.Chan:
    // compare len and elements instead of v1 == v2
}

func nonComparableKind(k reflect.Kind) bool {
    switch k {
    case reflect.Slice, reflect.Map, reflect.Func, reflect.Chan:
        return true
    }
    return false
}

When it happens

Trigger: Evaluating `p s1 == s2` where s1/s2 are slices, `p m1 == m2` for maps, `p f1 == f2` for funcs, or `p c1 == c2` for channels (all non-nil comparisons).

Common situations: Users expecting the debugger to deep-compare slices or maps the way a testing framework (reflect.DeepEqual) would; the debugger follows Go language rules instead.

Related errors

• shift count must not be negative
• invalid argument %s (type %s) for cap
• wrong number of arguments to len: %d
• invalid argument %s (type %s) for len
• wrong number of arguments to complex: %d

AI-assisted analysis of go-delve/delve@a23773e6c3 (2026-08-31). Data as JSON: /api/errors/1a432f43cf236307. Report an issue: GitHub.