proposal: strings: add First/Last function

[xrfang]

I encountered many times the following scenario:

func First(s string, n int) string {
    if len(s) <= n {
        return s
    }
    return s[:n]
}

This simple function try to get at most N-bytes from a string. Likewise, for at most N-bytes "tail" from a string:

func Last(s string, n int) string {
    if len(s) <= n {
        return s
    }
    return s[len(s)-n:]
}

These functions will many some programs easier to read, like the TrimPrefix/TrimSuffix functions.

[it512]

Left ， Right ？

[xrfang]

Name of the function is not important, it could be Prefix(), Suffix(), or LeftMost(), RightMost()

[dsnet]

For semi-related prior discussion, see #53510

[xrfang]

@dsnet I don't think my proposal has same intention as #53510. The key difference is, in #53510, @ianlancetaylor asked:

Should First and Last panic for an empty slice?

The answer is "yes, in the same way that s[0] panics for an empty slice.". But for this proposal, the key point is that I want to get the first (last) N bytes, if there is that much, or more, if the string is shorter, just give me all -- do NOT panic. This has exactly the same rational as "strings.TrimPrefix()".

So, comparing to #53510, this is not merely a syntactic sugar, it is a shorthand for, what I shink, a common logic.

[gophun]

I want to get the first (last) N bytes

Why bytes? Bytes are efficient as index positions into strings, usually determined via Index* functions, but they are not suitable for counting. The concept of 'count' doesn't have an obvious meaning in Unicode strings because there are multiple layers to consider.

[earthboundkid]

I'm afraid @gophun's objection is fatal. Anything you do (count bytes, count runes, count grapheme clusters) will be appropriate for some users and inappropriate for others. It does occur to me though that for counting bytes, you can now use min to make a one liner: s := mystr[:min(N, len(mystr))].

[seankhliao]

I believe these are equivalent to
s[:min(n, len(s))]
s[max(0, len(s)-n):]