flag: add and support implicit boolFlag interface

[gopherbot]

by brady:

In the flag module the following code:

if fv, ok := flag.Value.(*boolValue); ok { // special case: doesn't need an arg
  if has_value {
    if err := fv.Set(value); err != nil {
      return false, f.failf("invalid boolean value %q for  -%s: %v", value, name, err)
    }
  } else {
    fv.Set("true")
}

Ensures that its completely impossible to implement any kind of call back when a flag is
provided without value. Would it be possible to define an interface that has say a
"Default()" function, and use that here to cast/call so that its possible for
a third party flag to have access to this special case?

Examples of implementations where this is useful:

# Count the number of times -v is passed.
./count_dash_v -v -v -v -v

# Provide very specific help (and then exit)
./special_help --help_about_some_topic

etc.

If desired I can provide a patch as well, I figure this would be trivial to implement
outside of the inevitable naming war. =)

[robpike]

Comment 1:

I'm not sure it's valuable enough to bother. The flag package is intricate enough
already and without careful guarding flag packages become overfertilized. However, if (I
stress - if) one were to do this, a simpler, API-changeless way to do it would be to
change the if statement to see if the flag is some kind of boolean, as opposed to
explicitly boolValue. Then the client could use the existing interfaces to provide the
desired behavior.
reflect.ValueOf(flag.Value).Elem().Kind() == reflect.Bool

Status changed to Thinking.

[rsc]

Comment 3:

If we were going to do this, I would not do it by tying the value to
the underlying type. It should be something declared explicitly by the
flag. For example, a flag implementation could optionally satisfy
type BoolFlag interface {
    IsBoolFlag() bool
}

[rsc]

Comment 4:

1. Add interface from comment #3 as boolFlag (unexported).
2. Add IsBoolFlag method to boolVar implementation.
3. Document in flag.Value comment:
//
// If a Value has an IsBoolFlag() bool method returning true,
// the command-line parser makes -name equivalent to -name=true
// rather than using the next command-line argument.

Labels changed: added priority-later, removed priority-triage.

Status changed to Accepted.

[rsc]

Comment 5:

Labels changed: added size-m.

[rsc]

Comment 6:

Labels changed: added suggested.

[gopherbot]

Comment 7 by rickarnoldjr:

Fixed by https://golang.org/cl/6944064

[rsc]

Comment 8:

This issue was closed by revision a6d986b.

Status changed to Fixed.