go/printer: Fprintf uses different settings than go fmt

[kevinburke]

Please answer these questions before submitting your issue. Thanks!

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

1.7

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

All

What did you do?

If I write a Go file with go/printer.Fprintf, structs get printed like this - I've replaced the tab characters with \t so it's clear what is going on.

type User struct{
\tName\t\tstring
\tLongConfig\tuint
}

If you immediately run go fmt ./... on the file, some of the tabs are replaced by spaces:

type User struct{
\tName        string
\tLongConfig  uint
}

go/printer.Fprintf uses the following default printer.Config argument:

return (&Config{Tabwidth: 8}).Fprint(output, fset, node)

However, go fmt uses this config argument:

const (
    tabWidth    = 8
    printerMode = printer.UseSpaces | printer.TabIndent
)

// ...

res, err := format(fileSet, file, sourceAdj, indentAdj, src, printer.Config{Mode: printerMode, Tabwidth: tabWidth})

What did you expect to see?

I expect the two "default" functions to print the file the same way

What did you see instead?

One prints with tabs in the middle and one doesn't. This led to a lot of thrash when running anything automated over the codebase; I noticed this first as part of the investigation for Shyp/bump_version#1.

Far more people run go fmt than write AST parsing/hacking functions, so it seems like the fix, if there is to be one, should be to have printer.Fprintf match the go fmt behavior.

[kevinburke]

I've signed the CLA and happy to submit a fix, but not sure if this would count as a breaking API change, or is desirable.

[rakyll]

/cc @griesemer

[quentinmit]

Changing this seems reasonable to me. In general, we shouldn't make breaking changes to the format, but since it would be to match what go fmt already does, I think I agree that it's a net win.

[kevinburke]

@griesemer - thoughts? Maybe I should just submit a patch?

[griesemer]

@kevinburke My apologies but I haven't looked into this yet as it doesn't seem urgent. I will look into this in the next couple of days.

[griesemer]

@kevinburke I agree there's a discrepancy.

The intention behind the printer's default settings was to produce tab-aligned output. gofmt's fine-tuning (using blanks for all alignment but indentation) came later. (There was a long period of trial and error with gofmt's settings before it eventually settled to what we have now).

I'd be somewhat reluctant to change the settings here simply because I don't know how much code depends on what's here now.

Instead, I'd recommend that you use go/format (format.Node) instead of printer.Fprint. It has the same signature and it supposed to produce gofmt-compliant output.

If you're ok with this answer I'll close this issue. Otherwise, please raise your objections. Thanks.

[bradfitz]

Maybe documentation somewhere saying a short version of that?

[gopherbot]

CL https://golang.org/cl/29790 mentions this issue.