slices: add CloneFunc function

[apocelipes]

Proposal Details

I found that there is a common pattern when copying data from slices:

var ss []T
for _, v := range s {
	if conditions {
		ss = append(ss, v)
	}
}

I did a simple scan of the "golang/go" code base, and found more than 30 places of this pattern. For examples:

• archive/tar/common.go#L47-L52
• errors/join.go#L29-L36
• go/build/build.go#L1093-L1098
• log/slog/record.go#L118-L123
• net/lookup.go#L534-L543

So I propose to add a simple function "CloneFunc". "CloneFunc" may look like this:

// CloneFunc copies any elements from s for which f returns true
func CloneFunc[S ~[]E, E any](s S, f func(E) bool) S {
	ret := make([]E, 0, len(s)) // Maybe it's not worth doing this
	for i := range s {
		if f(s[i]) {
			ret = append(ret, s[i])
		}
	}
	return ret
}

There are similar functions in other languages:

• C++: std::copy_if
• Java: Stream.filter
• JavaScript: Array.prototype.filter

I personally think "CloneFunc" can make the code simpler, taking "net/lookup.go" as an example:

func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
	...
-	filteredAddrs := make([]*SRV, 0, len(addrs))
-	for _, addr := range addrs {
-		if addr == nil {
-			continue
-		}
-		if !isDomainName(addr.Target) {
-			continue
-		}
-		filteredAddrs = append(filteredAddrs, addr)
-	}
+	filteredAddrs := slices.CloneFunc(addrs, func(addr *SRV) { return addr != nil && isDomainName(addr.Target) } )
	if len(addrs) != len(filteredAddrs) {
		return cname, filteredAddrs, &DNSError{Err: errMalformedDNSRecordsDetail, Name: name}
	}
	return cname, filteredAddrs, nil
}

"CloneFunc" may not be a good name, but it conforms to the naming rules of the slices APIs. Maybe calling it "Filter" would make more sense.

Maybe it is simpler to use loops directly in many cases, but I think it is better to add common patterns to the slices standard library, which can also reduce the difficulty of switching from other languages ​​to golang.

I'm happy to contribute a pull request with tests if this is considered a good idea.

[zephyrtronium]

The conventional name for this operation is filter. The name CloneFunc makes me think of map. And FWIW, both filter and map are special cases of foldMap.

[ianlancetaylor]

Thanks. Closing as dup of #61899 combined with #61898.