proposal: fmt: append decimals to whole-number floats when using '%#v' #26363

dave · 2018-07-13T04:11:02Z

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (`go version`)?

1.10.3

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (`go env`)?

darwin/amd64

What did you do?

fmt.Printf("%#v", float64(0.0))
Expected: 0.0
Actual: 0
https://play.golang.org/p/KkcUz1voJG4

As per the description in e97f407 I would expect to see the decimal point added for all floats. The %#v format string is intended to return valid Go syntax. In this case, the returned Go syntax represents an integer instead of a float.

It looks like the commit linked above forgot that the sharp flag is replaced with sharpV in the case of the v verb here...

There's more discussion of this here: dave/jennifer#39
Attn: @martisch

The text was updated successfully, but these errors were encountered:

dave · 2018-07-13T04:36:34Z

Hey @martisch I'd love to have a go at fixing this myself... Would that be ok with you?

martisch · 2018-07-13T05:15:58Z

Sure, happy if you send a fix with test case, if this is indeed agreed upon to need a fix (https://golang.org/doc/contribute.html).

Looking at it again It is not 100% clear to me if we should fix this and documentation breaking backwards compatibility or remove the mention of v in the code that handles # for floats.

Note that the behavior of printing 0 also seems to be in go1.4 and possibly earlier so not introduced by e97f407. #v is special and the documentation says "always print a decimal point for %e, %E, %f, %F, %g and %G;" so my commit message was inconsistent in that regard but from the code it should have applied to #v too.

Go language spec says "A floating-point literal is a decimal representation of a floating-point constant. It has an integer part, a decimal point, a fractional part, and an exponent part." (where some parts can be omitted) and fmt documentation says "%#v a Go-syntax representation of the value". Interestingly it only says of the value and not a literal of the type, so it would seem to me 0 is ok according to the documentation and does not force a 0.0 or 0. literal.

@robpike for further opinion if #v should always have a dot or leave current formatting to not break existing uses of #v (since at least go1.4).

I lean to keeping output as is and fix the code to not handle the unreachable case of verb v in conjunction with f.sharp.

dave · 2018-07-13T05:21:31Z

Yep I agree it could break backwards compatibility. It would definitely break the work-around I made for the bug here.

I'll leave you guys to make the call. Happy to fix if needed...

robpike · 2018-07-14T03:26:16Z

I think it's a bug and should be fixed.

Fixes golang#26363

gopherbot · 2018-07-14T07:28:02Z

Change https://golang.org/cl/123956 mentions this issue: fmt: fix incorrect format of whole-number floats when using %#v

dave · 2018-07-14T07:28:19Z

So there's a couple of corner cases that I've made assumptions:

Should 1.0 print 1.0 or 1.?
Although 1. is more compact, I think 1.0 looks like more natural Go syntax...

Should 1e6 print 1e+06 or 1.0e+06?
Since 1e+06 (which is the current output) will already be interpreted as a float, I don't think we want to change it.

Here's a PR: #26383

dave · 2018-08-28T08:16:11Z

Hey is there anything I need to do to keep this from going stale?

This fixes the unwanted behaviour where printing a zero float with the #v fmt verb outputs "0" - e.g. missing the trailing decimal. This means that the output would be interpreted as an int rather than a float when parsed as Go source. After this change the the output is "0.0". Fixes golang#26363

This fixes the unwanted behaviour where printing a zero float with the that the output would be interpreted as an int rather than a float when parsed as Go source. After this change the the output is "0.0". Fixes golang#26363

bcmills · 2018-10-10T20:59:29Z

As noted in #27634, a breaking change to fmt should go through the proposal process, bug or otherwise.

bcmills · 2018-10-11T12:21:10Z

See also #27634 (comment) for an argument that the existing behavior was correct (but admittedly not the clearest possible choice).

gopherbot · 2018-10-16T17:10:55Z

Change https://golang.org/cl/142597 mentions this issue: Revert "fmt: fix incorrect format of whole-number floats when using %#v"

Numbers without decimals are valid Go representations of whole-number floats. That is, "var x float64 = 5" is valid Go. Avoid breakage in tests that expect a certain output from %#v by reverting to it. To guarantee the right type is generated by a print use %T(%#v) instead. Added a test to lock in this behavior. This reverts commit 7c7cecc. Fixes #27634 Updates #26363 Change-Id: I544c400a0903777dd216452a7e86dfe60b0b0283 Reviewed-on: https://go-review.googlesource.com/c/142597 Run-TryBot: Filippo Valsorda <filippo@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>

rsc · 2018-10-17T17:45:15Z

Because this broke things, we rolled it back. We're going to leave things as is. Declining the proposal.

martisch self-assigned this Jul 13, 2018

martisch added the NeedsFix The path to resolution is known, but the work has not been done. label Jul 13, 2018

martisch removed their assignment Jul 13, 2018

martisch removed the NeedsFix The path to resolution is known, but the work has not been done. label Jul 13, 2018

martisch added this to the Go1.12 milestone Jul 13, 2018

martisch added the NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. label Jul 13, 2018

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

321e439

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

b6eae64

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

fceeadd

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

ec7b60b

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

4bbf5a5

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

3e3d85c

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

1e621a2

Fixes golang#26363

dave added a commit to dave/go that referenced this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v

f9ecdc5

Fixes golang#26363

dave mentioned this issue Jul 14, 2018

fmt: fix incorrect format of whole-number floats when using %#v #26383

Closed

ianlancetaylor added the NeedsFix The path to resolution is known, but the work has not been done. label Jul 14, 2018

gopherbot removed the NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. label Jul 14, 2018

dave mentioned this issue Jul 16, 2018

Improper float formatting dave/jennifer#39

Closed

gopherbot closed this as completed in 7c7cecc Aug 28, 2018

mark-rushakoff mentioned this issue Sep 10, 2018

fmt: %#v of integer value in float64 (fraction = 0) incorrectly has ".0" suffix #27598

Closed

zegl mentioned this issue Sep 12, 2018

fmt: 7c7cecc should be rolled back #27634

Closed

bcmills reopened this Oct 10, 2018

bcmills changed the title ~~fmt: v verb with sharp modifier incorrectly formats whole-number floats~~ proposal: fmt: v verb with sharp modifier incorrectly formats whole-number floats Oct 10, 2018

bcmills added Proposal NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. labels Oct 10, 2018

gopherbot removed the NeedsFix The path to resolution is known, but the work has not been done. label Oct 10, 2018

bcmills changed the title ~~proposal: fmt: v verb with sharp modifier incorrectly formats whole-number floats~~ proposal: fmt: append decimals to whole-number floats when using '%#v' Oct 10, 2018

rsc closed this as completed Oct 17, 2018

rsc added the Proposal-Declined label Oct 17, 2018

golang locked and limited conversation to collaborators Oct 17, 2019

gopherbot added the FrozenDueToAge label Oct 17, 2019

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proposal: fmt: append decimals to whole-number floats when using '%#v' #26363

proposal: fmt: append decimals to whole-number floats when using '%#v' #26363

dave commented Jul 13, 2018

dave commented Jul 13, 2018

martisch commented Jul 13, 2018 •

edited

dave commented Jul 13, 2018 •

edited

robpike commented Jul 14, 2018

gopherbot commented Jul 14, 2018

dave commented Jul 14, 2018 •

edited

dave commented Aug 28, 2018

bcmills commented Oct 10, 2018

bcmills commented Oct 11, 2018

gopherbot commented Oct 16, 2018

rsc commented Oct 17, 2018

proposal: fmt: append decimals to whole-number floats when using '%#v' #26363

proposal: fmt: append decimals to whole-number floats when using '%#v' #26363

Comments

dave commented Jul 13, 2018

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

dave commented Jul 13, 2018

martisch commented Jul 13, 2018 • edited

dave commented Jul 13, 2018 • edited

robpike commented Jul 14, 2018

gopherbot commented Jul 14, 2018

dave commented Jul 14, 2018 • edited

dave commented Aug 28, 2018

bcmills commented Oct 10, 2018

bcmills commented Oct 11, 2018

gopherbot commented Oct 16, 2018

rsc commented Oct 17, 2018

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

martisch commented Jul 13, 2018 •

edited

dave commented Jul 13, 2018 •

edited

dave commented Jul 14, 2018 •

edited