How to figure out if a flag's value was provided by the user or is the default

[josephschmitt]

I have a simple command-line wrapper around an API endpoint. One of my commands is an update command which takes a few flags, such as --name, --description, etc.

type UpdateCmd struct {
	Name string `kong:"optional"`
	Description string `kong:"optional"`
}

Ideally, I'd like for that update command to only mutate the values that were changed, and not touch anything else. However, kong only lets flags be values and not pointers. This means that if I run my update command without any flags, it'll populate the Name and Description struct fields with their default values of "", and my code has no real easy way to determine if the user intention was to not update that field, or if they want to wipe that field out.

Is there an easy-ish way to determine if a value was set because it was defaulted vs because the user explicitly set it to be that value?

My ideal use case would be:

# Only updates the name field
$ cli update --name "My cool name"

# Only updates the description
$ cli update --description "Some cool description"

However the way it works right now, excluding either flag sets that value to be an empty string. If I try this now, running

$ cli update --name "My cool name"

Will produce a go struct of the following

fmt.Printf("%+v", cmd) // &{Name:My cool name Description:}

And then calling my update API endpoint will now wipe out my description, which was not the user intent. If the user wanted to wipe out the description, I would expect them to have to do

$ cli update --name "My cool name" --description ""

The way I'd normally do this is by making these struct fields pointers, with a nil pointer meaning the value was never set, but kong doesn't allow flags to be pointers.

[alecthomas]

This is by design, so the behaviour won't be changing, but I think a helper function WasSetByUser() or something, would be useful.

[josephschmitt]

Yea that'd be great. Can you think of a way that I could do this in my app using kong's current API? Or is this something we'd need to add in to kong first?

[josephschmitt]

Actually, I was able to work around this by using a custom Mapper and having my flag be *string. But a WasSetByUser() would definitely be useful in the future!