fmt: verb override using tags on struct fields.

[gopherbot]

by BeMasher:

I find that often when I'm writing types I need to apply special fmt formatting to
specific fields in structs while leaving the rest as the default "v" fmt verb.
Implementing Stringer on the type becomes cumbersome when I only need to modify the
formatting for one or two fields in a struct.

For example the following type requires a tedious String function where adding or
removing fields requires modification in the type and the String function:

type Foo {
    Bar  []byte
    Baz1 uint32
    Baz2 uint32
    Baz3 uint32
    Baz4 uint32
    Baz5 uint32
}

func (f Foo) String() string {
    return fmt.Sprintf("{Bar:%s Baz1:%d Baz2:%d Baz3:%d Baz4:%d Baz5:%d}", f.Bar, f.Baz1, f.Baz2, f.Baz3, f.Baz4, f.Baz5)
}

One solution is to add a tag to the Bar field specifying the verb to use since it is the
only field that needs to be formatted differently from the standard "v" verb.
This allows me to use fmt functions that don't require a format string without
constantly having to update the String method.

type Foo {
    Bar  []byte `fmt:"s"`
    Baz1 uint32
    Baz2 uint32
    Baz3 uint32
    Baz4 uint32
    Baz5 uint32
}

Printing using fmt.Println (or other non-formatting) then produces the desired result
without needing to implement Stringer for the type.

Need to determine when tags will be used or ignored, particularly when a format string
is supplied.

[GeertJohan]

Comment 1:

Interesting idea. Giving it some thoughts.. I think the tag should contain the % for
completeness. So your example would then become `fmt:"%s"`. This will keep formats like
"% x" and "%0d" readable and will be more recognizable.

[cznic]

Comment 2:

Are we going to write this eventually?
type foo struct {
        bar qux `"fmt:whatever" "json:whatever" "xml:whatever" "bson:whatever" "mgo:whatever" "gob:whatever" "mydsl:whatever"`
        baz int ...
}

[adg]

Comment 3:

I pretty much only print structs when debugging. Is that what you use it for, too? If
so, I don't think this extra mechanism is worth it.

Labels changed: added feature, priority-someday, removed priority-triage, go1.2maybe.

Owner changed to @robpike.

Status changed to Thinking.

[gopherbot]

Comment 4 by BeMasher:

It makes sense to have the tag obey all the verb formatting like the "%" prefix
suggested.
Going through the fmt source, it looks like it would be reasonable to ignore tags when
either the go syntax "%#v" verb is used and any verb other than "v".
I've got most of the logic written already, just need to write some tests.

[robpike]

Comment 5:

After sleeping on it I've decided I don't like this. There are already two mechanisms to
control printing of values and, although perhaps less convenient, they cover the job.
Let's not push for more use of tagged structures.

Status changed to WontFix.