time: add Time.GoString method to help %#v [freeze exception]

[kevinburke1]

I have an array of time.Time objects that I retrieved from the database, and I would like to place them in a Go source file. As I understand it, there's not an easy way to do this without a helper method.

• fmt.Sprintf("%#v\n", t) prints out something like:

  time.Time{wall:0x0, ext:63724924180, loc:(*time.Location)(nil)}

  which not only makes it difficult to determine what time is represented - I can't do math on 63724924180 in my head to determine whether this is yesterday or today, but also can't be embedded directly in a Go program because wall and ext and loc are private variables.

• I could use t.Format(time.RFC3339) on each item to get an array of strings, but then these need to be re-Parse'd to get a time.Time back, with appropriate error handling.

It could be nice to have either a (time.Time) GoString() string method or a GoString format constant which would print out a format that could be embedded in a Go source file.

For example

fmt.Sprintf(t.Format(time.GoString))

could yield

"time.Date(2020, time.February, 31, 23, 59, 59, 0, nil)"

or similar. I am not sure whether having time.Time implement the GoStringer interface - and change the format printed by %#v - would violate the Go compatibility promise, though I doubt the current wall, ext, loc format is very useful for most people.

The trickiest part of this would be printing Location, which could be implemented by printing out the pointer address for non-nil, non-UTC, non-Local locations - it's no worse than what's currently done.

I suppose we'd also lose some data about the monotonic-ness of the time measurement in question but if you're trying to put this in source code you likely don't care about that bit.

[kevinburke1]

Ah, t.Format(time.GoString) would not work since we can only do string formatting there and that would likely be insufficient for printing the Location part so it does not need error handling. Still we could satisfy the GoStringer interface.

[OneOfOne]

I'm not really sure that needs to be in the stdlib, it's trivial to implement without t.Format

https://play.golang.org/p/Lzzbd55VBeH

[kevinburke1]

If your struct is

type A struct {
     B time.Time
}

a := &A{B: time.Now()}
fmt.Printf("%#v\n", a)

Suddenly you need to either implement your custom format string for everything, just so you can get at the time.Time, or hack up something like Printf("&A{B: %s}\n", toGoTime(a.B)) to rebuild the struct formatting around your custom element formatting.

More to the point this is why the GoStringer interface exists.

[OneOfOne]

I see your point, you can still easily wrap it but yeah it'd be nice.

https://play.golang.org/p/OhAGAR6bPs4

[rsc]

This mostly makes sense, but it would have to hide the monotonic time if any.

[kevinburke1]

Hi, ping here, I think this would be a very helpful change for Go 1.16, and I would love to work on this if there is interest.

[gopherbot]

Change https://golang.org/cl/267017 mentions this issue: time: add GoString method

[kevinburke1]

Hi, now that the 1.17 source tree is open, I would be interested in getting feedback on this proposal. I submitted CL 267017 as an idea for how this feature could be implemented, that could hopefully be used as a jumping off point.

[rsc]

This seems reasonable (still).
Will list in the minutes this week.

Does anyone object to adding this?

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[kevinburke1]

The stickiest point I think is how to represent a *time.Location value. The most common cases for a *time.Location - nil, UTC, or Local - are all easy to handle. However the other cases - fixed zones, or Locations that represent values in the timezone database, like "America/Los_Angeles" - are more tricky.

1. One way would be to try to represent it the same way we represent nested pointers in a struct.

   (*time.Location)(0xbadbcde)

   As I understand the runtime, it would be difficult to get the pointer address without importing either fmt (which imports reflect) or reflect directly, to get and then print the pointer address. The time package is not allowed to import reflect as Russ pointed out.

   I suppose we could special case time.Time inside of the fmt package when someone asks for a GoString representation, since fmt is allowed to import reflect, but that seems less than ideal.

2. Another way would be to convert it to a fixed zone, with the offset determined by the instant represented by the time. You lose information here in some cases, because you would be converting something like "America/Los_Angeles" into a fixed time zone with fixed offset.

   time.FixedZone(loc.name, determineOffset(loc))

   If loc is the America/Los_Angeles timezone, determineOffset would return 8*60*60 = 28800 for a time instant in the winter, and 7*60*60 = 25200 for a time instant in the summer when daylight savings time is active.

3. Another way would just be to print a time.Location value with a lowercase name property. This wouldn't compile, but would be no worse than what we print today for %#v, which also does not compile. We'd need to build in logic to escape strings in case the name field has a " or other characters in it that need escaping.

   time.Date(2020, time.February, 31, 23, 59, 59, 0, &time.Location{name: "America/Los_Angeles"}) 
   ./prog.go:9:79: cannot refer to unexported field 'name' in struct literal of type time.Location

4. Another way would be to replace it with time.LoadLocation, which would also not compile because LoadLocation returns two values. I don't think this is a great idea, but thought I'd mention in the interest of completeness.

   time.Date(2020, time.February, 31, 23, 59, 59, 0, time.LoadLocation("America/Los_Angeles"))
   ./prog.go:9:81: multiple-value time.LoadLocation() in single-value context

Of these, I think (3) is best. I don't think it's possible to implement (1) unless I am missing some aspect of the runtime that would make it possible to retrieve a pointer without importing the reflect package.