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

[Ross O'Brien]

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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