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

Charlie Monroe charlie at charliemonroe.net
Thu Jul 21 14:00:25 CDT 2016 

> On Jul 21, 2016, at 5:46 PM, 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***. 

In which case it should be even more confusing to you:

let str = "Hello"
str.characters.starts(with: "".characters) // true
str.hasPrefix("") // false

> 
> 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 <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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>> 
> 
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