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

[David Hart]

This is the best argument: lets not make the behaviour surprising to people coming from other languages out there.

> On 21 Jul 2016, at 04:57, Jaden Geller via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This discussion is getting out of control. Both of these functions have mathematical precedent as well as consistent behaviors in other languages. Observe:
> 
> Python: `"hello world".startswith("")` => `True`
> Java: `"hello world".startsWith("")` => `true`
> JavaScript: `"hello world".startsWith("")` => `true`
> Ruby: `"hello world".start_with? ""` => `true`
> Rust: `"hello world".starts_with("")` => `true`
> Go: `strings.HasPrefix("hello world", "")` => `true`
> 
> It's pretty hard to argue against this. Even if you think these other languages are wrong, Swift must regard the empty String as a prefix to be consistent with itself.
> 
> `str.hasPrefix(String(str.characters.prefix(0)))` => `false` ?!
> 
> I would expect every prefix of `str` to return true as an argument to `str.hasPrefix`…
> 
>> On Jul 20, 2016, at 6:49 PM, Ted F.A. van Gaalen via swift-evolution <swift-evolution at swift.org <mailto: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 <mailto: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 <mailto:glessard at tffenterprises.com>> wrote:
>>>> 
>>>>> 
>>>>>> On 20 juil. 2016, at 14:21, Xiaodi Wu <xiaodi.wu at gmail.com <mailto: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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