proposal: strings: add Cut

[ainar-g]

In many projects I've seen code like this:

idx = strings.Index(username, "@")
if idx != -1 {
  name = username[:idx]
} else {
  name = username
}

idx = strings.LastIndex(address, "@")
if idx != -1 {
  host = address[idx+1:]
} else {
  host = address
}

I think this operation—getting a prefix or suffix based on a substring—is common enough to think about adding helpers for them. So I propose to add functions PrefixUntil and SuffixAfter (names up for debate) to package strings. Something like:

// PrefixUntil returns the prefix of the string s until the first
// occurrence of the substring substr, excluding the substring itself.
// It returns s if substr was not found.
func PrefixUntil(s, substr string) (prefix string) {
	var idx = Index(s, substr)
	if idx == -1 {
		return s
	}

	return s[:idx]
}

// SuffixAfter returns the suffix of the string s after the last
// occurrence of the substring substr, excluding the substring itself.
// It returns s if substr was not found.
func SuffixAfter(s, substr string) (suffix string) {
	var idx = LastIndex(s, substr)
	if idx == -1 {
		return s
	}

	return s[idx+len(substr):]
}

So that the code could be rewritten as:

name = strings.PrefixUntil(username, "@")

host = strings.SuffixAfter(address, "@")

[ianlancetaylor]

It would help if you could identify some cases in the standard library, and/or in other popular Go packages, where the new functions would be used. That is, give some supporting information for "in many projects I've seen...." Thanks.

[ainar-g]

@ianlancetaylor A simple grep query shows lots, actually. Here's one from package encoding/xml:

if i := strings.LastIndex(prefix, "/"); i >= 0 {
	prefix = prefix[i+1:]
}

Here's one from path:

if i := strings.LastIndex(path, "/"); i >= 0 {
	path = path[i+1:]
}

net/url:

i := strings.LastIndex(authority, "@")
if i < 0 {
	host, err = parseHost(authority)
} else {
	host, err = parseHost(authority[i+1:])
}

net/http (IndexByte, but the meaning is the same):

if i := strings.IndexByte(resp.Status, ' '); i != -1 {
	statusCode = resp.Status[:i]
}

go/types:

if i := strings.LastIndex(name, "/"); i >= 0 {
	name = name[i+1:]
}

test/fixedbugs/issue32778.go, a copy of the code from the original issue, which in turn, I assume, is a version of the code the reporter of that issue saw:

if i := strings.LastIndexByte(string(n), '.'); i >= 0 {
	return Name(n[i+1:])
}
return Name(n)

Lots of that in test/run.go.

I discovered these using:

$ grep -A 5 -e 'strings.\(Last\)\?Index(' -r

And then sifting the results manually.

[networkimprov]

This looks like a variation on strings.SplitN(str, "@", 2). So maybe SplitFirst & SplitLast?

[ainar-g]

@networkimprov Those functions don't really split (turn one into many) strings. And the proposed logic is different from that of the Split.* functions. Perhaps FirstPrefix and LastSuffix.

[jimmyfrasche]

@ainar-g I believe these would be equivalent

name = strings.PrefixUntil(username, "@")
name, _ = strings.SplitFirst(username, "@")

[martisch]

If I should rather Split off the below into its own proposal I can but it seems the frequency of both patterns (prefix, suffix and partition) together makes for a potential better outlook to be relevant enough for addition.

Adding more general strings.SplitFirst and strings.SplitLast:

Much code I have optimised in parsing functions involves splitting around an identifier and uses Index and LastIndex.
so a key, value, ok = strings.SplitFirst(kv, separator) (and same for Last) would be nice and would cover the use cases of strings.PrefixUntil and SuffixAfter too.

Why ok as a return? To distinguish the case of separator appearing in the source string or not. Knowing value == "" does not provide that.

I can give Googlers references to internal CLs. Here is one for std lib:
https://go-review.googlesource.com/c/go/+/231738/1/src/encoding/asn1/common.go

Why not strings.Split(N)? these allocate a return slice and there is no strings.SplitLastN.

Alternative: strings.Partition like in Python:

Another take on this could be to align this with str.partition from Python:
https://docs.python.org/3/library/stdtypes.html#str.partition

str.partition(sep)
Split the string at the first occurrence of sep, and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing the string itself, followed by two empty strings.

So we could discuss strings.Partition(s, sep string) (first, split, tail) and strings.PartitionLast(s, sep string) (head, split, last string) as an alternative with similar semantics as SplitFirst and SplitLast.

[rogpeppe]

If something like this is accepted (and on balance I'm in favour), I'd prefer at least one variant that has a single return value so that it can be used directly in expressions.

[rsc]

There may be an opportunity here, but it's a pretty subtle thing to get right as a general function.
For example, the example above is a bit difficult if the string has no @ sign:

name = strings.PrefixUntil(username, "@")
host = strings.SuffixAfter(address, "@")

parsing "rsc" will end up with name="rsc", host="rsc", which is not what you want.

The Python partition function is more like what you want, in that it distinguishes "separator is present" from "separator is not". But it's a bit awkward to use too since there are no tuples to index like in Python, and allocating a slice defeats the purpose.

There are many helper functions in strings already. Why is this one important, and - even more importantly - can we handle all the needs with a single function and still be convenient?

[martisch]

The reason why I like a function that does a pair split is that I have optimized many parsing functions where you want to split off one element of a delimited list in a string without allocating and at the same time getting the rest of the string without the delimiter and split of element. Having a std library function do this that doesnt allocate a return slice would help readability:
https://go-review.googlesource.com/c/go/+/231738/1/src/encoding/asn1/common.go

returning elem, separator, tail or elem, tail, ok as 3 return values does not need a slice or allocation.

[rsc]

We certainly write code to do this operation all the time. Even just grepping for 'Index.* >=' in the Go tree I find plenty of code like:

if i := strings.Index(line, "#"); i >= 0 {
	line = line[:i]
}

and

if i := strings.Index(raw, "@"); i >= 0 {
	pattern, rawVers = raw[:i], raw[i+1:]
	if strings.Contains(rawVers, "@") || rawVers == "" {
		return nil, fmt.Errorf("invalid module version syntax %q", raw)
	}
}

If we had a function like:

func Cut(s, sep string) (before, after string, ok bool) {
    if i := strings.Index(s, sep); i >= 0 {
        return s[:i], s[i+len(sep):], true
    }
    return s, "", false
}

then snippets like the above would become:

line, _, _ = strings.Cut(line, "#")

if pattern, rawVers, ok := strings.Cut(raw, "@"); ok {
    // no need for: pattern, rawVers = raw[:i], raw[i+1:]
    ...
}

It does seem like it comes up a lot. What do people think? Should we add this (single) Cut function?

Update: Edited to return (before, after, ok) instead of (before, sep, after), as suggested by @nightlyone.

[nightlyone]

Since we pass in the separator, one could also return an OK bool instead of the separator.

For this I would suggest

Cut(s, sep string) (before, after string, ok bool)