I often do nil checks on interfaces in places like constructors that may or may
not have received optional parameters. In some cases as a parameter that may be
nil if no implementation is required, in some cases where a functional option
setting the interface was not given. Typically this looks something like:
func Walk(ctx context.Context, cfg Config) (chan protocol.FileInfo, error) {
w := walker{cfg}
if w.CurrentFiler == nil {
w.CurrentFiler = defaultCurrentFiler{}
}
…
}
where the field is set to some default implementation when nil. It’s also not
uncommon to have nil implementations of that interface, especially in tests:
type mockCurrentFiler struct{}
func (*mockCurrentFiler) CurrentFile(string) whatever {
return nil
}
var testCurrentFiler *mockCurrentFiler // nil
…
cfg.CurrentFiler = testCurrentFiler // == nil in the new regime
Walk(ctx, cfg)
This would break, in a surprising way as the default implementation would be
used instead of the given mock implementation. Clearly the test can be
refactored:
cfg.CurrentFiler = &mockCurrentFiler{} // not nil
but yes, the change would break code.
I’m not saying that having this change in Go 2 would necessarily be a bad idea,
but the current Go 1 behavior makes sense in Go 1 and there is definitely code
that depends on it.
//jb
On 3 Nov 2017, at 11:09, oju...@gmail.com<mailto:oju...@gmail.com> wrote:
Yes, Ayan, you are quite correct. There are a lot of comparisons to check if an
error is nil. It has it's own FAQ entry: https://golang.org/doc/faq#nil_error
Sincerely, what I am after is an example of a situation where the solution
proposed by Dave Cheney would create a problem, like "See here. In this
situation the proposed change would break my code."
When comparing error to nil, Dave's change would work just fine. That FAQ entry
would not be needed anymore.
On Friday, November 3, 2017 at 7:31:47 AM UTC-2, Ayan George wrote:
On 11/03/2017 05:23 AM, oju...@gmail.com<javascript:> wrote:
> Could somebody please point me to any example of comparing an
> interface to nil, in production code? Thanks.
>
Does checking if a variable of type error is nil count?
-ayan
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