proposal: fmt: add Printlnf, Fprintlnf, etc

[leighmcculloch]

Add print functions to fmt that support formatting and append a new line, e.g. Printlnf, Fprintlnf, etc.

The fmt package has these printing functions today:

• Fprint
• Fprintf
• Fprintln
• Print
• Printf
• Println
• Sprint
• Sprintf
• Sprintln

The ln functions are convenient when printing lines to stderr and stdout. The f functions are required for formatting but have no ln equivalent.

It is a pretty common occurrence for me to write or edit code that uses Println and Fprintln, then to have a need to add a formatted parameter to that output. Without fail I rename the function to Printf and forget to include a \n at the end.

If f variants are added for all the ln functions, there would be functions Printlnf, Fprintlnf, etc.

The intended uses of the existing functions would become:

• Print - Print, no new line appended, no formatting.
• Printf - Print, with formatting.
• Println - Print, new line appended.
• Printlnf - Print, with formatting, new line appended.

A prototype implementation is available at https://4d63.com/fmt.

Example:

func Printlnf(format string, a ...interface{}) (n int, err error)
func Fprintlnf(format string, a ...interface{}) (n int, err error)

code := 400
message := "error message"

fmt.Fprintlnf(os.Stderr, "error: code %d, %s", code,  message)
fmt.Fprintln(os.Stderr, "resending request")
// Output:
// error: code 400, error message
// resending request

It may also make sense for consistency to add the same variation of functions for each scan function in the fmt package.

Proposal template:

• Would you consider yourself a novice, intermediate, or experienced Go programmer?
  Experienced

• What other languages do you have experience with?
  Java, Ruby, C#, C, JavaScript

• Would this change make Go easier or harder to learn, and why?
  A little easier.

• Has this idea, or one like it, been proposed before?
  Yes, We should have fmt.Printfln series of functions in package #31214.

  • If so, how does this proposal differ?
    It's identical. The author abandoned their proposal.

• Who does this proposal help, and why?
  Anyone printing lines.

• What is the proposed change?
  See above.

  • Please describe as precisely as possible the change to the language.
    See above.

  • What would change in the language spec?
    Nothing.

  • Please also describe the change informally, as in a class teaching Go.
    See above.

• Is this change backward compatible?
  Yes

  • Breaking the Go 1 compatibility guarantee is a large cost and requires a large benefit.

• Show example code before and after the change.
  See above.

• What is the cost of this proposal? (Every language change has a cost).

  • How many tools (such as vet, gopls, gofmt, goimports, etc.) would be affected?
    None.
  • What is the compile time cost?
    Little.
  • What is the run time cost?
    None.

• Can you describe a possible implementation?
  See above.

  • Do you have a prototype? (This is not required.)
    Yes. See https://4d63.com/fmt.

• How would the language spec change?
  It wouldn't.

• Orthogonality: how does this change interact or overlap with existing features?
  It is modeled after existing print functions in fmt package.

• Is the goal of this change a performance improvement?
  No.

  • If so, what quantifiable improvement should we expect?
  • How would we measure it?

• Does this affect error handling?
  No.

  • If so, how does this differ from previous error handling proposals?

• Is this about generics?
  No.

  • If so, how does this differ from the the current design draft and the previous generics proposals?

[mvdan]

My intuition is that you win very little by adding more variants, and this will be very confusing to people trying to read code - particularly those new to Go. It's not that hard to write fmt.Fprintf("%x\n", foo) rather than fmt.Fprintlnf("%x", foo). And you get used to the \n suffix.

[leighmcculloch]

And you get used to the \n suffix.

This is what I thought would happen for me, but it hasn't. I still forget to add \n when using the f variations, especially when changing code from using Println to Printf to add formatting.

this will be very confusing to people trying to read code

Could you expand on why this will be very confusing?

[mvdan]

I still forget to add \n when using the f variations, especially when changing code from using Println to Printf to add formatting.

Mistakes can happen. I imagine you're just as likely to forget to add ln than you are to forget to add \n, though.

Could you expand on why this will be very confusing?

More options to choose from, mainly. Especially since all these func names vary only by one or two characters. We already have quite a lot of variants, and they exist for a good reason. More variants when it's just a very minor convenience doesn't seem worth it to me.

[adonovan]

I don't like this idea. The Println variant differs from Print and Printf in two ways, one obvious (the newline) and one subtle: its treatment of arguments that are strings with leading or trailing whitespace. For example:

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

fmt.Sprint(" a ", " b ") == " a  b " // two spaces
fmt.Sprintln(" a ", " b ") == " a   b \n" // three spaces

What does the lnf variant do? I cannot guess, and nor should I have to.

[leighmcculloch]

It might make it clearer if the function was named fln, or Printfln, so it's clearer that this is a Printf function with a new line appended.

@adonovan Would that make the fln variant clearer?

What does the lnf variant do?

The f variant does not add spaces because it receives only a single string for formatting, so the fln variant should not either.

[adonovan]

@adonovan Would that make the fln variant clearer?

No. The problem is that behavior of lnf (or fln) is not the composition f and ln. It is yet another thing readers must commit to memory.

[rsc]

#46190 (comment) is the major reason not to do this. Println is not Print+\n. It changes other things that make no sense to change in Printf.

[magical]

@rsc @adonovan If we think of Printf as Print(Sprintf(...)) and Printlnf as Println(Sprintf(...)) then there's no confusion. Print(ln)'s weird behaviour with multiple arguments doesn't apply, and the only relevant difference is the fact that Println appends a newline. I don't think that alone is sufficient reason to reject this proposal.

I'm somewhat in favor of this proposal. Like @leighmcculloch, I often forget \n in my format strings, particularly when converting a Println call to a Printf call. Having Printlnf available would probably help alleviate that. However, i'm not sure if the minor benefits are worth the new API.

[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

[leighmcculloch]

i'm not sure if the minor benefits are worth the new API

The main problem this solves is addressing the mistake some of are repeatedly making with missing \n in print calls. To that end, adding a lnf variation for every existing fmt function would probably be unnecessary. I think the proposal could be limited to adding Printlnf and Fprintlnf. Originally I was thinking other functions, such as Sprintlnf, should be added for consistency, but that is a much larger API change. If we limit the proposal to only adding Printlnf and Fprintlnf, the scope of the new API introduced is much smaller and focused on the problem.

[bvisness]

I too sometimes forget to put \n in my format strings, but those mistakes are always balanced out by having too many newlines (using Println when I should have used Print for example). I always end up gravitating back to Printf for everything, and if I make a couple mistakes, oh well.

There are clear reasons in this thread why these proposed functions would be confusing, and the design decisions around Print and Println make it pretty clear that if precise formatting matters to you, you should just be using Printf.

I don't think the inevitable confusion could ever be worth it for a problem as trivial as forgetting a newline.

[rsc]

@rsc @adonovan If we think of Printf as Print(Sprintf(...)) and Printlnf as Println(Sprintf(...)) then there's no confusion.

But that's not what Println means.