[swift-evolution] Change Request: Make myString.hasPrefix("") and myString.hasSuffix("") return true

[Tony Allevato]

A deeper problem is that String is already internally inconsistent because
of the special case in hasPrefix/Suffix:

    String("abc").characters.prefix(0) == ""

which should imply that "abc".hasPrefix("") == true.

The existing implementation of hasPrefix/Suffix inherited from NSString is
a bug, not a feature, and I can cite it as a source of actual bugs in
production code that I have personal experience with. The correct
implementation being proposed here is backed by formal set/sequence theory.

Unfortunately, just fixing hasPrefix/Suffix isn't enough—since the String
type in Swift gets many of its primitive operations via Foundation.NSString
bridging, it will still inherit a lot of wrong behavior from there:

    "abc".rangeOfSubstring("") currently returns nil, when it should return
Range(startIndex: 0, endIndex: 0), because that is the location of the
earliest range where "" is found.
    "abc".containsString("") currently returns false when it should return
true.

Since Dave Abrahams mentioned earlier that Swift 4 would re-work strings,
I'm hopeful that a lot of this inherited behavior from Foundation will be
replaced by Swift-native code that does the right thing.


On Thu, Jul 21, 2016 at 8:46 AM Ted F.A. van Gaalen via swift-evolution <
swift-evolution at swift.org> wrote:

>
>
> There are many empty strings in that string. In fact, there are infinite
> empty strings between each character, before the string, and after the
> string. Observe:
>
> "" + "Don’t Panic: Please read Hitchhiker’s Guide to the Galaxy 42"
> "" + "" + "Don’t Panic: Please read Hitchhiker’s Guide to the Galaxy 42"
> "" + "" + "" + "Don’t Panic: Please read Hitchhiker’s Guide to the Galaxy
> 42"
> "" + "" + "" + "" + "Don’t Panic: Please read Hitchhiker’s Guide to the
> Galaxy 42"
> etc, and I didn't even get past the first character!
>
> Wel, maybe I am not intelligent enough to comprehend that,
> or maybe it’s just a matter of definition/convention..
>
> Again, to me a string is ***just a row of characters***.
>
> therefore, concatenating empty strings (that do not contain any
> characters)  with other strings have no effect: .
> for example:
>
>        let res = "" + "" + "" + "" + “The Art Of Learning To Fly”
>
> after that:
>
>      res == “The Art Of Learning To Fly”
>
> and:
>
>      res.count == “The Art Of Learning To Fly”.count
>
> Regardless what in many  other programming languages  is done;
> I prefer the Objective jC NSString hasPrefix(“") way of handling this,
> which always returns False,e because a row of characters
> is contiguous, without empty “” in between, leading or trailing.
>
> However, we don’t seem to share the same opinion, about this sorry.
> nothing more to say about that, I guess.
>
> TedvG
>
>
>
> On Jul 20, 2016, at 6:49 PM, Ted F.A. van Gaalen via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Don’t Panic !
>
> At the risk of seeing things in a somewhat trivial perspective,
> combined with an almost complete absence of abstraction:
>
> Note that to relatively simple persons like me:
>
> String instances are just rows of characters (when not empty, of course)
>
> There are only two kinds of Strings:
>
> 1. empty Strings, which do not contain amy characters at all
>
>   and
>
> 2.  Strings containing 1 or more characters.
>
> Ergo ad Infinitum :
>
> Empty Strings do not occur in Strings that contain characters.
>
>
> I’d say, please try to find possible empty strings
> that might perhaps be embedded e.g. in the string below:
>
> “Don’t Panic: Please read Hitchhiker’s Guide to the Galaxy 42”
>
>
> With all due respect:
> This might void the discussion below :o)
>
> I have nothing against Mathematics as long
> as it is applicable.
>
>
> Kind Regards
> Ted
>
>
>
> To the question of whether any given string has the empty string as prefix:
> yes it does. This is a correct answer, and returning true is a correct
> behaviour.
>
> To the question of how many times the empty string occurs in a string: yes,
> this can be infinite. "a" == "a" + "" == "a" + "" + "" == "a" + "" + "" +
> "" == "a" + "" + "" + "" + "" == ... etc.. Concatenating an empty string,
> like adding zero or multiplying by zero for a numerical value, can be done
> infinitely many times without making a difference.
>
> However, there's correctness and convenience. For example, every integer
> can be expressed as a multiple of prime factors. 1 is technically a prime
> number - it's divisible by 1 and itself - but for convenience we say it
> isn't a prime number, because if it isn't, every integer can be expressed
> uniquely as a multiple of prime factors, whereas if it is, there are an
> infinite number of such expressions for each integer.
>
> There may be many algorithms which rely on an empty prefix returning false.
> If hasPrefix and hasSuffix are corrected, those algorithms should be
> altered to recognise that correction. For example, if breaking up a string
> using the empty string as a separator, it seems sensible that the sequence
> of substrings would never contain consecutive empty strings.
>
> On Wed, Jul 20, 2016 at 11:58 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> I'd run this by someone who actually knows math, but afaik there are
> finitely many empty strings in any given string.
>
> How many e's are in any given string? (Ignoring Unicode issues for now,)
> for each index in the string's indices, form a substring one character in
> length starting at that index and compare the value of that substring to e.
>
> How many empty strings are in any given string? For each index in the
> string's indices, form a substring zero characters in length starting at
> that index and compare the value of that substring to an empty string.
>
>
>
> On Wed, Jul 20, 2016 at 17:35 Guillaume Lessard <
> glessard at tffenterprises.com> wrote:
>
>
> On 20 juil. 2016, at 14:21, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> Doesn't your second argument undermine your first? If it's a trivial
>
> solution and one rarely ever considers empty strings when invoking
> `hasPrefix`, then returning the technically correct result must be a
> trivial departure in behavior.
>
> I specifically used an example where the trivial solution (y=0 instead of
> y=exp(x)) is a pitfall.
>
> How many empty strings are contained in any given string?
> If the answer is infinitely many, it sounds like a pitfall to me.
>
> Cheers,
> Guillaume Lessard
>
>
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