Go: proposal: fmt: feature request: add Error, which corresponds to Errorf as Print corresponds to Printf

[brackendawson]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.9 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/alandaws"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/06/jjq25lws0cbcxv37ns0hb_mm0000gn/T/go-build733634811=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Nothing, feature request

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

What did you expect to see?

n/a

What did you see instead?

n/a

A common use of fmt.Errorf is to wrap a generic error returned from an upstream library so you can see where it occurred in your code:

x, err := something.DoThing()
if err != nil {
    return nil, fmt.Errorf("Failed to do thing here: %s", err)
}

Concatenation in other variadic functions in fmt seems to be slightly faster than processing format strings, and in the case of wrapping errors, needs less syntax.

I could write:

x, err := something.DoThing()
if err != nil {
    return nil, fmt.Error("Failed to do thing here: ", err)
}

The differences are only marginal, but it does fit with the pattern of a lot of the other functions in fmt, one that's just variadic and one that takes a format string.

Using Dave Cheney's errors.Wrap is possibly a better approach altogether: https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully

Do people think a fmt.Error function is valuable?

[ianlancetaylor]

The advantage of fmt.Error over errors.New seems very minor to me. Just change , to +.

[brackendawson]

You do have to also change err to err.Error(), but I agree that is equivalent for performance, and only slightly longer.

errors.New("Something: " + err.Error())
fmt.Errorf("Something: %s", err)
fmt.Error("Something: ", err)

[rsc]

It's already a little confusing that fmt.Errorf returns an error and log.Errorf prints a line of text to stderr. And .Error means other things in other APIs. Let's not compound this by adding fmt.Error here.

[rsc]

Note that you can already do errors.New(fmt.Sprint(...)) if you really want to avoid the format string.