proposal: cmd/gofmt: add option to insert missing end-of-line commas in program

[rhysd]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.7.4 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/rhysd/.go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.4_1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.4_1/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sv/mr3y8ytd7gzfdww8gxx568z80000gn/T/go-build764880956=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

First, save below code as blah.go

package main

func main() {
	a := []int{
		1,
		2
	}
}

Then execute below command

$ gofmt blah.go

What did you expect to see?

Automatically fix trailing comma at 2 (line 6, column 9) and formatting has been done successfully.

What did you see instead?

Formatting failed with below error

blah.go:6:4: missing ',' before newline in composite literal

I believe this is because gofmt can't format a code which contains syntax error. Trailing comma such as line 6 in above is a syntax error in Go. So gofmt cannot fix this. However, we tend to forget adding trailing comma when expose one line code to multi-line code. If gofmt can fix it automatically, we no longer need to take care about trailing commas.

I guess implementing this is not so difficult. When parsing failed, then fix the source and retry to parse it again.

[ALTree]

gofmt is a code formatter, I don't think it should be required to handle invalid go code, much less change it to correct errors.

[mvdan]

It also goes down a slippery slope. What syntax errors should be fixed? Where do you draw the line?

[rhysd]

How about adding a tool like a goimports? It is a code fixer for import statements.

[ALTree]

gofixtrailingcommas ? Mmh...

Honestly I'm not sure it would meet the bar for inclusion in x/tools/cmd; but you can always publish it as an open source project. I don't think it'll be used as widely as goimports though, this is a much smaller problem.

[rhysd]

I agree that adding new tool like this is bike-shedding... But is there any chance to add more generic tool like go fix? For example, go fix imports does the same as goimports. go fix trailling-comma fixes commas, ...

[rhysd]

but you can always publish it as an open source project.

Hmm... I can create a fixer which fixes something before passing a code to gofmt. I think I should try it (although parsing twice and spawning a process for gofmt may be not good for performance).

[rhysd]

How about adding a new option to ignore parse error if no bad node is in AST? I tried modifying gofmt code as below.

https://github.com/rhysd/gofmtrlx

rhysd/gofmtrlx@76c93b2

I found that there is no bad node in AST even if trailing comma error occurs. So, as gofmt is formatter, it can ignore the parse error, just format the code even if parse error is contained when there is no bad node in AST. How do you think?

[haya14busa]

btw, gofmt already trims trailing , in function call.

$ gofmt -d test.go
diff test.go gofmt/test.go
--- /tmp/gofmt596094646 2017-02-04 23:43:27.384131425 +0900
+++ /tmp/gofmt073783965 2017-02-04 23:43:27.384131425 +0900
@@ -7,7 +7,7 @@
                3,
        }

-       f(1, 2, )
+       f(1, 2)
 }

I think it's reasonable to add missing trailing comma to some extent considering this behavior.
Especially if we can draw a reasonable line with detecting ast.Bad* node.
It's also really useful.

[mvdan]

@haya14busa note that what you're describing there is transforming a valid program into another valid program.

[haya14busa]

Oh, that's true. I didn't know it's valid syntax because I'm always using gofmt before compile.

But my main point is, if go/parser can correctly created ast node, why not using it instead of discarding it with trivial error.

[mattn]

@rhysd I think that gofmt ignore the syntax error is not good. I'm not sure but, how about ignoring the error in your editor or something using gofmt? For example, vim-go may be possible to ignore the output of gofmt by filtering that error. missing ',' before newline in composite literal

[rhysd]

@mattn If ignoring the error, I can't know why formatting failed and nothing was formatted. I don't want to stop formatting.

[rhysd]

After all, what I want to say is what @haya14busa said (thanks @haya14busa). Even if there is a parse error, Go's parser can parse it. gofmt is not a syntax checker. IMO gofmt should be able to format every code which parser can parse without bad AST node.

[rhysd]

Please note that I don't say this should be default behavior. But I feel there should be an option for this behavior.