cmp: add Or

[earthboundkid]

An extremely common string operation is testing if a string is blank and if so replacing it with a default value. I propose adding First(...strings) string to package strings (and probably an equivalent to bytes for parity, although it is less useful).

// First returns the first non-blank string from its arguments.
func First(ss ...string) string {
	for _, s := range ss {
		if s != "" {
			return s
		}
	}
	return ""
}

Here are three example simplifications from just archive/tar because it shows up first alphabetically when I searched the standard library:

archive/tar diff

diff --git a/src/archive/tar/reader.go b/src/archive/tar/reader.go
index cfa50446ed..bc3489227f 100644
--- a/src/archive/tar/reader.go
+++ b/src/archive/tar/reader.go
@@ -136,12 +136,8 @@ func (tr *Reader) next() (*Header, error) {
 			if err := mergePAX(hdr, paxHdrs); err != nil {
 				return nil, err
 			}
-			if gnuLongName != "" {
-				hdr.Name = gnuLongName
-			}
-			if gnuLongLink != "" {
-				hdr.Linkname = gnuLongLink
-			}
+			hdr.Name = strings.First(gnuLongName, hdr.Name)
+			hdr.Linkname = strings.First(gnuLongLink, hdr.Linkname)
 			if hdr.Typeflag == TypeRegA {
 				if strings.HasSuffix(hdr.Name, "/") {
 					hdr.Typeflag = TypeDir // Legacy archives use trailing slash for directories
@@ -235,13 +231,8 @@ func (tr *Reader) readGNUSparsePAXHeaders(hdr *Header) (sparseDatas, error) {
 	hdr.Format.mayOnlyBe(FormatPAX)
 
 	// Update hdr from GNU sparse PAX headers.
-	if name := hdr.PAXRecords[paxGNUSparseName]; name != "" {
-		hdr.Name = name
-	}
-	size := hdr.PAXRecords[paxGNUSparseSize]
-	if size == "" {
-		size = hdr.PAXRecords[paxGNUSparseRealSize]
-	}
+	hdr.Name = strings.First(hdr.PAXRecords[paxGNUSparseName], hdr.Name)
+	size := strings.First(hdr.PAXRecords[paxGNUSparseSize], hdr.PAXRecords[paxGNUSparseRealSize])
 	if size != "" {
 		n, err := strconv.ParseInt(size, 10, 64)
 		if err != nil {
diff --git a/src/archive/tar/writer.go b/src/archive/tar/writer.go
index 1c95f0738a..e9c635a02e 100644
--- a/src/archive/tar/writer.go
+++ b/src/archive/tar/writer.go
@@ -188,10 +188,7 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 		var name string
 		var flag byte
 		if isGlobal {
-			name = realName
-			if name == "" {
-				name = "GlobalHead.0.0"
-			}
+			name = strings.First(realName, "GlobalHead.0.0")
 			flag = TypeXGlobalHeader
 		} else {
 			dir, file := path.Split(realName)

[seankhliao]

Duplicate of #14423

[earthboundkid]

@seankhliao, good find, but that issue was frozen before the modern proposal process existed. Either that issue should be reopened and put through the proposal process or this one should be unclosed, but I don't think it's fair to call it a duplicate when the old one was never actually evaluated.

[seankhliao]

The idea was clearly evaluated in the previous issue and declined.
The decisions we made before the proposal process are just as valid.

[earthboundkid]

I don't think it's fair to call 5 comments "clearly evaluated." The reception was mixed. Abbgrade was for it. Minux was against it. Bradfitz was neutral to positive on the idea if there was more data.

It ends with @griesemer saying,

This is not an issue, this is a feature request. Please discuss this first on one of the popular Go forums (mailing list, etc.).

I don't think he would have said "go discuss it somewhere else" if that discussion was precluded from having an effect because the issue was permanently closed once and for all. I think the idea was "go discuss it more and if it comes up again we can take another look." Now we have a formal process, so it's time to take a look. :-)

[seankhliao]

It was quite clear it doesn't belong in strings, and the natural place for it be in now, slices, also has the similar idea being declined in #52006

[earthboundkid]

It doesn't work in slices because it would need a T comparable, which is confusing, or to be a find func, which as you note was already declined. Just because it could be a generic doesn't mean it should be. :-) I've been using my personal stringutils.First for years and for me it's above the bar to get it into the standard library. Maybe I'm wrong, but I think it's worth having a discussion.

[ianlancetaylor]

I agree that the earlier issue didn't get a full proposal review. We can do it again.

That said, finding some more examples would help justify adding this.

And, in general the strings and bytes package are parallel. What would this look like in bytes, and would anybody use that variant?

[earthboundkid]

That said, finding some more examples would help justify adding this.

I've been using a version of First for at least three years that I can recall, and I'm up to 19 uses in a 13,000 line project. It's pretty routinely useful for me. (It might go back further, and I've just forgotten the history of it.)

Going back to the examples from archive/tar above, I think there's a readability gain in hdr.Name = strings.First(gnuLongName, hdr.Name) and name = strings.First(realName, "GlobalHead.0.0"), because you can tell quickly tell what the preferred value is and what's the fallback default, whereas in the old code the first example was set with if gnuLongName != "" and the second was set with if realName == "".

What would this look like in bytes, and would anybody use that variant?

I suppose it should be First(...[]byte) []byte, but I agree that it is unlikely to be used much, since the main use is to set a default for a string value.