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

[Dave Abrahams]

on Tue Jul 19 2016, "Ted F.A. van Gaalen" <tedvgiosdev-AT-gmail.com> wrote:

> Hi Dave
>
> “true” ? am I going nuts ? :o) 
>
> var str = "Hello, playground"
>
> print( str.hasPrefix("”)) // case 1 : false
>
> print( str.hasSuffix("”)) // case 2 : false
>
> print("" == “a” )          // case 3 : false
>
> Currently, all cases above evaluate to “false”
> i think that is correct, 

I don't know what to tell you.  It may seem intuitively correct to you,
but others in the thread have laid out the reasons why it is not
mathematically correct behavior.  One other way of rephrasing it: to get
`false` for str.hasPrefix(""), you actually need special-case code in
hasPrefix to check for the empty string, and the caller may well also
need special-case code to handle the fact that the result is not
mathematically consistent with other cases on the continuum.  Doing
things that way doesn't work in practice for real programs.

> because:
>
> How can an empty string be a prefix or suffix value?
> as there is no empty string present in a non-empty string. 
>
> Note that if case 1 and case 2 would evaluate to “true”, 
> it would conflict with case 3. 
>
> Can’t imagine that case 3 should in future also result in “true” 
>
> ??
>
> -----
>
> Also I hope that changes to String functionality
> for Swift 4 are not backward breaking changes
> even the more for string handling, because Strings 
> are heavily used in most apps. 
>
> I am firmly convinced that all future releases of Swift 
> should compile Swift 3 and higher source files without
> any changes 100 % flawlessly! This prevents early diminishing 
> of Swift’s popularity, especially with those building large
> codebases using Swift. 
>
> I’ve started a thread about this a week ago,
> however no one found this important enough to
> share their opinions with me yet, or were too busy with
> other subjects to do so.
>
> Increasingly I have dreams, me
> programming complete apps in Smalltalk
> and then automagically generate 
> an macOS, tvOS or iOS runtime app of it.
>
> (I have also dreams of this world becoming
> a nice and peaceful placebut that is 
> beyond the context of this forum)    
>
> Kind Regards
> TedvG 
>
> www.speyer.de
>
>> on Mon Jul 18 2016, Kevin Nattinger <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> I agree, true is definitely the expected behavior. In particular, it
>>> seems absurd to me that `a.hasPrefix(b)` and `a.hasSuffix(b)` could be
>>> false when `a == b` is true.
>> 
>> I expect to be reworking Strings for Swift 4, and this is one of the
>> many things we plan to address.
>> 
>> -- 
>> Dave
>> 
>> 
>

-- 
Dave