Dan Ingalls is of course a big NAME in the world of Smalltalk, but the stated reason for changing the behaviour of #beginsWith: and #endsWith: makes no sense. We have many ways to define a partial order on strings. x <= y iff y beginsWith: x x <= y iff y endsWith: x x <= y iff y includesSubCollection: x x <= y iff y includesSubSequence: x These things are supposed to obey laws: if a beginsWith: b , c then a beginsWith: b if a endsWith: b , c then a endsWith: c if a includesSubCollection: b , c then a includesSubCollection: b and a includesSubCollection: c if a includesSubSequence: b , c then a includesSubSequence: b and a includesSubSequence: c. We also expect the usual rules of equality to hold. So (1) a beginsWith: a (2) a = '' , a (3) THEREFORE a beginsWith: '' (1) a endsWith: a (2) a = a , '' (3) THEREFORE a endsWith: '' (1) a includesSubCollection: a (2) a = '' , a (3) THEREFORE a includesSubCollect: '' Reasoning about strings (as values) gets enormously more complicated if the operations do not follow simple sensible rules, and having '' be the least string under these orderings and having '' be a prefix and a suffix of any string is essential if the rules are going to be simple and coherent. '' is to strings (and more generally empty sequences are to sequences) pretty much what 0 is to integers. Denying that 'abc' beginsWith: '' is *structurally* just like denying that 0 <= 123. Now as it happens I *can* see a use for versions of #beginsWith: and #endsWith: that diverge from the ones we have, but *this* is not where they need to diverge. a beginsWithGraphemesOf: b iff a asGraphemes = b asGraphemes , c asGraphemes for some c, where s asGraphemes returns a sequence of strings each of which is a maximally long grapheme cluster, such that concatenating s asGraphemes recovers s. That is, #beginsWithGraphemesOf: and #endsWithGraphemesOf: would respect the Unicode Text Segmentation boundaries. But s beginsWithGraphemesOf: '' would still need to be true. The thing is, in practice you often DON'T KNOW whether a potential affix is empty or not. Here are some of my test cases. testTestData "Ensure that the sample string has no duplicates." [(Set withAll: string) size = stringSize] assert. testBeginsWith "Test that every prefix of the sample IS a prefix of it." 0 to: stringSize do: [:n | [string beginsWith: (string copyFrom: 1 to: n)] assert]. testEndsWith "Test that every suffix of the sample IS a suffix of it." 0 to: stringSize do: [:n | [string endsWith: (string copyFrom: stringSize - n + 1 to: stringSize)] assert]. testIndexOfSubCollectionAtBeginning "Test that every prefix of 'abcd' is found at the beginning." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string indexOfSubCollection: s startingAt: 1. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert]. testIndexOfSubCollectionAtEnd "Test that every proper suffix of the sample is found at the end." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string indexOfSubCollection: s startingAt: 1. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert]. testLastIndexOfSubCollectionAtBeginning "Test that every proper prefix of the sample is found at the beginning." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string lastIndexOfSubCollection: s startingAt: stringSize. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert]. testLastIndexOfSubCollectionAtEnd "Test that every suffix of the sample is found at the end." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string lastIndexOfSubCollection: s startingAt: stringSize. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert]. testOccurrencesOfEmptyCollection "Test that the empty string occurs at the beginning, at the end, and in between every pair of adjacent characters." [(string occurrencesOfSubCollection: '') = (stringSize + 1)] assert. testOccurrencesOfUniqueParts "Test that unique parts occur as many times as they should." |repeated| repeated := string , string , string. 1 to: stringSize do: [:start | start to: stringSize do: [:finish | |s n| s := string copyFrom: start to: finish. n := string occurrencesOfSubCollection: s. [n = 1] assert. n := repeated occurrencesOfSubCollection: s. [n = 3] assert]]. On Fri, 22 Apr 2022 at 13:57, David T. Lewis <lewis@mail.msen.com> wrote:

Hi Richard,

(CC squeak-dev list, replies to the relevant list please)

On Thu, Apr 21, 2022 at 12:07:32AM +1200, Richard O'Keefe wrote:

...

I've just tracked down a nasty little problem porting some code to Pharo. As a result, I have added to the comments in my own versions of these methods. beginsWith: aSequence "Answer true if aSequence is a prefix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' beginsWith: '' + VisualWorks, Dolphin, astc, GNU ST (where the method is called #startsWith:) and VisualAge (where the method is called #wbBeginsWith:) agree than EVERY sequence begins with an empty prefix. - Squeak and Pharo agree that NO sequence begins with an empty sequence. # ST/X chooses compatibility with Squeak, heaving a big unhappy sigh, and adds #startsWith: to have something sensible to use. Now ST/X *thinks* it is compatible with VW, though it isn't, so I wonder if this was a bug that VW fixed and Squeak didn't? astc goes with the majority here. This is also compatible with Haskell, ML, and with StartsWith in C# and startsWith in Java." ^self beginsWith: aSequence ignoringCase: false

endsWith: aSequence "Answer true if aSequence is a suffix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' endsWith: ''. + VisualWorks, Dolphin, astc, GNU ST, and VisualAge (where the method is called #wbEndsWith:) agree that EVERY sequence ends with an empty suffix. - Squeak and Pharo agree that NO sequence ends with an empty suffix. # ST/X chooses compatibility with the majority, apparently unaware that this makes #beginsWith: and #endsWith: inconsistent. astc goes with the majority here. This is also compatible with Haskell, ML, C#, and Java." ^self endsWith: aSequence ignoringCase: false

Does anyone have any idea - why Squeak and Pharo are the odd ones out? - why anyone thought making #beginsWith: and #endsWith:, um, "quirky" was a good idea (it's pretty standard in books on the theory of strings to define "x is a prefix of y iff there is a z such that y = x concatenated with z")

The Squeak behavior was introduced in November 1998 by Dan Ingalls in conjunction with some VM improvements that he was doing, and was included in the Squeak2.2 release at that time.

Prior to that update, Squeak did this: 'abc' beginsWith: '' ==> true 'abc' endsWith: '' ==> true

For Squeak2.2 and later it is: 'abc' beginsWith: '' ==> false 'abc' endsWith: '' ==> false

Pharo presumably inherits this from Squeak.

I am attaching Dan's original change set from the early Squeak update stream so you can see the context.

The change set comment says that "endsWith:, beginsWith:, and match have been rewritten to take advantage of this considerably faster method" (referring to a new primitive that Dan had added to the VM in this change set).

...

I was about to try to file a bug report for the first time, then realised that maybe other people don't think this IS a bug.

I don't think it is a bug. I can't think of a case where it makes sense to say that a string of characters "begins with" or "ends with" a string that contains nothing.

In any case, that's the history of the change in Squeak and Pharo as best as I can reconstruct it.

HTH, Dave

I have now raised it as an issue on the issue tracker Issue #11165 <https://github.com/pharo-project/pharo/issues/11165> in https://github.com/pharo-project/pharo/issues/11165 <https://github.com/pharo-project/pharo/issues/11165> Best, Kasper

On 26 Apr 2022, at 12.11, Steffen Märcker <merkste@web.de> wrote:

I can only fully agree to Richard's explanation. Also, not having the empty string pre/suf-fixing every string breaks the free monoid that describes concatenation of strings and forms the basis of regular expressions. In effect, this will lead to subtle inconsistencies. Hence, I consider this a bug rather than a feature.

Kind regards, Steffen

Kasper Osterbye schrieb am Dienstag, 26. April 2022 14:50:51 (+02:00):

I have now raised it as an issue on the issue tracker

Issue #11165 <https://github.com/pharo-project/pharo/issues/11165> in https://github.com/pharo-project/pharo/issues/11165 <https://github.com/pharo-project/pharo/issues/11165> If I may suggest a solution it will be to: add two new methods - prefixedBy: and suffixedBy: to handle the empty prefix/suffix correctly add comments to beginsWith: and endsWith: referring to the two new methods

The problem is that there is client code which depend on the (wrong) implementation of the beginsWith and endsWith methods.

Making the code correct is "untwisting the rope". Keeping the code wrong and adding a bizarre workaround is "twisting the rope tighter". The former is the right direction. On Fri, Apr 29, 2022 at 12:15 AM Steffen Märcker <merkste@web.de> wrote:

Hi Kasper,

I've thought about that approach too. But then asked myself whether it is more likely that there is code that relies on this bug than code where this went unnoticed and is therefore broken. What do the other think about that matter and the fix in Squeak?

Best, Steffen

Kasper Osterbye schrieb am Donnerstag, 28. April 2022 19:03:07 (+02:00):

Kasper Osterbye schrieb am Dienstag, 26. April 2022 14:50:51 (+02:00):

I have now raised it as an issue on the issue tracker

Issue #11165 <https://github.com/pharo-project/pharo/issues/11165> in https://github.com/pharo-project/pharo/issues/11165

If I may suggest a solution it will be to:

- add two new methods - *prefixedBy: * and *suffixedBy:* to handle the empty prefix/suffix correctly - add comments to *beginsWith: * and *endsWith: * referring to the two new methods

The problem is that there is client code which depend on the (wrong) implementation of the beginsWith and endsWith methods.

I discovered that my own code-base was inconsistent due to sometimes following Pharo and sometimes not. I've just spent the last day making it consistent and you would not BELIEVE how many methods suddenly became simpler. I finally understood why some of the test cases I had for XPath were broken. XPath specifies that the empty string is a prefix and suffix of every string. Perhaps better support for XPath may have some persuasive value. (Though support for XPath collations will be a long way off. There's a LOT of other Unicode support I need to develop first.) On Tue, 26 Apr 2022 at 22:12, Steffen Märcker <merkste@web.de> wrote:

I can only fully agree to Richard's explanation. Also, not having the empty string pre/suf-fixing every string breaks the free monoid that describes concatenation of strings and forms the basis of regular expressions. In effect, this will lead to subtle inconsistencies. Hence, I consider this a bug rather than a feature.

Kind regards, Steffen

Richard O'Keefe schrieb am Samstag, 23. April 2022 02:37:48 (+02:00):

Dan Ingalls is of course a big NAME in the world of Smalltalk, but the stated reason for changing the behaviour of #beginsWith: and #endsWith: makes no sense.

We have many ways to define a partial order on strings. x <= y iff y beginsWith: x x <= y iff y endsWith: x x <= y iff y includesSubCollection: x x <= y iff y includesSubSequence: x These things are supposed to obey laws: if a beginsWith: b , c then a beginsWith: b if a endsWith: b , c then a endsWith: c if a includesSubCollection: b , c then a includesSubCollection: b and a includesSubCollection: c if a includesSubSequence: b , c then a includesSubSequence: b and a includesSubSequence: c.

We also expect the usual rules of equality to hold. So (1) a beginsWith: a (2) a = '' , a (3) THEREFORE a beginsWith: ''

(1) a endsWith: a (2) a = a , '' (3) THEREFORE a endsWith: ''

(1) a includesSubCollection: a (2) a = '' , a (3) THEREFORE a includesSubCollect: ''

Reasoning about strings (as values) gets enormously more complicated if the operations do not follow simple sensible rules, and having '' be the least string under these orderings and having '' be a prefix and a suffix of any string is essential if the rules are going to be simple and coherent.

'' is to strings (and more generally empty sequences are to sequences) pretty much what 0 is to integers. Denying that 'abc' beginsWith: '' is *structurally* just like denying that 0 <= 123.

Now as it happens I *can* see a use for versions of #beginsWith: and #endsWith: that diverge from the ones we have, but *this* is not where they need to diverge. a beginsWithGraphemesOf: b iff a asGraphemes = b asGraphemes , c asGraphemes for some c, where s asGraphemes returns a sequence of strings each of which is a maximally long grapheme cluster, such that concatenating s asGraphemes recovers s. That is, #beginsWithGraphemesOf: and #endsWithGraphemesOf: would respect the Unicode Text Segmentation boundaries. But s beginsWithGraphemesOf: '' would still need to be true.

The thing is, in practice you often DON'T KNOW whether a potential affix is empty or not. Here are some of my test cases.

testTestData "Ensure that the sample string has no duplicates." [(Set withAll: string) size = stringSize] assert.

testBeginsWith "Test that every prefix of the sample IS a prefix of it." 0 to: stringSize do: [:n | [string beginsWith: (string copyFrom: 1 to: n)] assert].

testEndsWith "Test that every suffix of the sample IS a suffix of it." 0 to: stringSize do: [:n | [string endsWith: (string copyFrom: stringSize - n + 1 to: stringSize)] assert].

testIndexOfSubCollectionAtBeginning "Test that every prefix of 'abcd' is found at the beginning." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string indexOfSubCollection: s startingAt: 1. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testIndexOfSubCollectionAtEnd "Test that every proper suffix of the sample is found at the end." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string indexOfSubCollection: s startingAt: 1. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testLastIndexOfSubCollectionAtBeginning "Test that every proper prefix of the sample is found at the beginning." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string lastIndexOfSubCollection: s startingAt: stringSize. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testLastIndexOfSubCollectionAtEnd "Test that every suffix of the sample is found at the end." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string lastIndexOfSubCollection: s startingAt: stringSize. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testOccurrencesOfEmptyCollection "Test that the empty string occurs at the beginning, at the end, and in between every pair of adjacent characters." [(string occurrencesOfSubCollection: '') = (stringSize + 1)] assert.

testOccurrencesOfUniqueParts "Test that unique parts occur as many times as they should." |repeated| repeated := string , string , string. 1 to: stringSize do: [:start | start to: stringSize do: [:finish | |s n| s := string copyFrom: start to: finish. n := string occurrencesOfSubCollection: s. [n = 1] assert. n := repeated occurrencesOfSubCollection: s. [n = 3] assert]].

On Fri, 22 Apr 2022 at 13:57, David T. Lewis <lewis@mail.msen.com> wrote:

...

Hi Richard,

(CC squeak-dev list, replies to the relevant list please)

On Thu, Apr 21, 2022 at 12:07:32AM +1200, Richard O'Keefe wrote:

...

I've just tracked down a nasty little problem porting some code to Pharo. As a result, I have added to the comments in my own versions of these methods. beginsWith: aSequence "Answer true if aSequence is a prefix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' beginsWith: '' + VisualWorks, Dolphin, astc, GNU ST (where the method is called #startsWith:) and VisualAge (where the method is called #wbBeginsWith:) agree than EVERY sequence begins with an empty prefix. - Squeak and Pharo agree that NO sequence begins with an empty sequence. # ST/X chooses compatibility with Squeak, heaving a big unhappy sigh, and adds #startsWith: to have something sensible to use. Now ST/X *thinks* it is compatible with VW, though it isn't, so I wonder if this was a bug that VW fixed and Squeak didn't? astc goes with the majority here. This is also compatible with Haskell, ML, and with StartsWith in C# and startsWith in Java." ^self beginsWith: aSequence ignoringCase: false

endsWith: aSequence "Answer true if aSequence is a suffix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' endsWith: ''. + VisualWorks, Dolphin, astc, GNU ST, and VisualAge (where the method is called #wbEndsWith:) agree that EVERY sequence ends with an empty suffix. - Squeak and Pharo agree that NO sequence ends with an empty suffix. # ST/X chooses compatibility with the majority, apparently unaware that this makes #beginsWith: and #endsWith: inconsistent. astc goes with the majority here. This is also compatible with Haskell, ML, C#, and Java." ^self endsWith: aSequence ignoringCase: false

Does anyone have any idea - why Squeak and Pharo are the odd ones out? - why anyone thought making #beginsWith: and #endsWith:, um, "quirky" was a good idea (it's pretty standard in books on the theory of strings to define "x is a prefix of y iff there is a z such that y = x concatenated with z")

The Squeak behavior was introduced in November 1998 by Dan Ingalls in conjunction with some VM improvements that he was doing, and was included in the Squeak2.2 release at that time.

Prior to that update, Squeak did this: 'abc' beginsWith: '' ==> true 'abc' endsWith: '' ==> true

For Squeak2.2 and later it is: 'abc' beginsWith: '' ==> false 'abc' endsWith: '' ==> false

Pharo presumably inherits this from Squeak.

I am attaching Dan's original change set from the early Squeak update stream so you can see the context.

The change set comment says that "endsWith:, beginsWith:, and match have been rewritten to take advantage of this considerably faster method" (referring to a new primitive that Dan had added to the VM in this change set).

...

I was about to try to file a bug report for the first time, then realised that maybe other people don't think this IS a bug.

I don't think it is a bug. I can't think of a case where it makes sense to say that a string of characters "begins with" or "ends with" a string that contains nothing.

In any case, that's the history of the change in Squeak and Pharo as best as I can reconstruct it.

HTH, Dave

-- Gesendet mit Vivaldi Mail. Laden Sie Vivaldi kostenlos von vivaldi.com herunter.

I like proverbs. One of mine is "zero is also a number." Way back in the 1980s the University in my home town was given a new IBM mainframe by IBM. The Computer Centre wrote a archival program for it that scanned each user's minidisk (= fixed partition) for files that had not been used for a while and logged them to a master file, then ran a job that moved the files to a tape. One day everything went horribly wrong. It turned out that the reason was very simple, but so alien that nobody had noticed when they read the documentation. It was the kind of thing that was so unbelievable you couldn't even *see* it when it was hinted at in front of you. Consider the following process: (1) create-or-replace a file. (2) write N records to it. (3) close the file. Simple. Obvious. And wrong. Because step (1) didn't actually *happen* until step (3), and if N = 0, it didn't happen then either. In that IBM mainframe operating system, you literally could not create an empty file. So when one day there weren't any files that needed moving to tape (because they had all been done the previous day), the process that was expected to write an empty list of files simply left the previous day's list unchanged. Result: thousands of multipage error reports when the archiver tried to move files that weren't there any more. All because someone at IBM had not been able to conceive of a file with no records! I hope the analogy is clear. But it gets better. If you wrote a file in C on that system, it was in effect line-buffered. Your text got written to the file when (and only when) you wrote a newline character. So if you imitated common MS-DOS practice and *began* lines with a newline, the last line of your file would never be written out. The equivalent of s := FileStream write: 'foo text a'. s nextPutAll: 'Hello world!'. s close. would write no records, and thus not create or replace the file. (Because a file with N = 0 newline characters is another of those N = 0 things that cannot exist!) I had to redesign the I/O library for a certain programming language that ran on a lot of Unix systems, on VAX/VMS, and we *thought* was running well through the C library on IBM mainframes. All sorts of finicky little details to deal with N=0. So at the end of a file, if nothing had been written, we wrote an empty record, unless you turned that option off. At the end of each record, if nothing had been written, we wrote a space. Yes, that N = 0 case was another problem. It has been over 30 years since I had anything to do with IBM mainframes, but some experiences leave scars. I note that in Pharo 9.0, there is a subtle difference between SequenceableCollection>>copyReplaceAll:with: and String>>copyReplaceAll:with: and the latter is not consistent with itself. Let x and y be Strings. x copyReplaceAll: '' with: y is always x, except when x is empty, in which case it is y. Let x and y be Arrays. x copyReplaceAll: #() with: y is always x. This is not the subtle difference I mentioned above. *That* difference is that the string version takes linear time (assuming fast search) while the general version takes quadratic time (also assuming fast search). The ANSI specification for #copyReplaceAll:with: is quite clear: Answer a new collection with the elements of the receiver in their original order, except where a subsequence in the receiver matches targetElements. A subsequence in the receiver is said to match the elements of targetElements if: 1. They have the same number of elements. 2. For all indices of the subsequence, the element in the subsequence at a given index is equivalent to the element in targetElements at the same index. By this definition, an empty sequence has matches in any sequence. I'm not advancing this as evidence that I'm right about #beginsWith: and #endsWith:, because the following text in the standard makes it clear that for some reason, empty sequences were literally UNTHINKABLE for the framers of the ANSI Smalltalk standard, at least in the context of search. I'm mentioning it as evidence that for many people, zero is the hardest number to think of. I've finally realised why some people stubbornly cling to the idea that the natural numbers begin with 1, not zero (as if the Peano axioms are not a thing). If you are thinking of the natural numbers in terms of *multiplication* (and primes, and so on), well, the positive integers are a cancellative monoid with identity 1, but not if you include 0. If you are thinking of the natural numbers in terms of *addition*, then the non-negative integers are a cancellative monoid with identity 0. So there are two *different* monoids, for one of which including 0 would be many kinds of trouble, and for the other of which excluding 0 would be absurd and awkward. Strings are cancellative monoids under concatenation with '' as the identity. (Don't ask me about Unicode.) So why is '' so hard to think about? I don't think it is just Strings. In functional programming, it was surprisingly hard to teach students to remember the empty list. (For myself, I got into the habit of writing the base case of a recursive function *first* to avoid the mistake of leaving out the base case.) I suspect that there are other places with surprising behaviour for 'trivial' cases. On Fri, 29 Apr 2022 at 13:35, David T. Lewis <lewis@mail.msen.com> wrote:

FYI it was fixed in Squeak a couple of days ago:

http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-April/220089.htm...

Richard O'Keefe's point of view was overwhelmingly supported, notably including this post:

http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-April/220038.htm...

Dave

On Sat, Apr 23, 2022 at 12:37:48PM +1200, Richard O'Keefe wrote:

...

Dan Ingalls is of course a big NAME in the world of Smalltalk, but the stated reason for changing the behaviour of #beginsWith: and #endsWith: makes no sense.

We have many ways to define a partial order on strings. x <= y iff y beginsWith: x x <= y iff y endsWith: x x <= y iff y includesSubCollection: x x <= y iff y includesSubSequence: x These things are supposed to obey laws: if a beginsWith: b , c then a beginsWith: b if a endsWith: b , c then a endsWith: c if a includesSubCollection: b , c then a includesSubCollection: b and a includesSubCollection: c if a includesSubSequence: b , c then a includesSubSequence: b and a includesSubSequence: c.

We also expect the usual rules of equality to hold. So (1) a beginsWith: a (2) a = '' , a (3) THEREFORE a beginsWith: ''

(1) a endsWith: a (2) a = a , '' (3) THEREFORE a endsWith: ''

(1) a includesSubCollection: a (2) a = '' , a (3) THEREFORE a includesSubCollect: ''

Reasoning about strings (as values) gets enormously more complicated if the operations do not follow simple sensible rules, and having '' be the least string under these orderings and having '' be a prefix and a suffix of any string is essential if the rules are going to be simple and coherent.

'' is to strings (and more generally empty sequences are to sequences) pretty much what 0 is to integers. Denying that 'abc' beginsWith: '' is *structurally* just like denying that 0 <= 123.

Now as it happens I *can* see a use for versions of #beginsWith: and #endsWith: that diverge from the ones we have, but *this* is not where they need to diverge. a beginsWithGraphemesOf: b iff a asGraphemes = b asGraphemes , c asGraphemes for some c, where s asGraphemes returns a sequence of strings each of which is a maximally long grapheme cluster, such that concatenating s asGraphemes recovers s. That is, #beginsWithGraphemesOf: and #endsWithGraphemesOf: would respect the Unicode Text Segmentation boundaries. But s beginsWithGraphemesOf: '' would still need to be true.

The thing is, in practice you often DON'T KNOW whether a potential affix is empty or not. Here are some of my test cases.

testTestData "Ensure that the sample string has no duplicates." [(Set withAll: string) size = stringSize] assert.

testBeginsWith "Test that every prefix of the sample IS a prefix of it." 0 to: stringSize do: [:n | [string beginsWith: (string copyFrom: 1 to: n)] assert].

testEndsWith "Test that every suffix of the sample IS a suffix of it." 0 to: stringSize do: [:n | [string endsWith: (string copyFrom: stringSize - n + 1 to: stringSize)] assert].

testIndexOfSubCollectionAtBeginning "Test that every prefix of 'abcd' is found at the beginning." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string indexOfSubCollection: s startingAt: 1. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testIndexOfSubCollectionAtEnd "Test that every proper suffix of the sample is found at the end." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string indexOfSubCollection: s startingAt: 1. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testLastIndexOfSubCollectionAtBeginning "Test that every proper prefix of the sample is found at the beginning." 1 to: stringSize do: [:n | |s i t| s := string copyFrom: 1 to: n. i := string lastIndexOfSubCollection: s startingAt: stringSize. [1 = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testLastIndexOfSubCollectionAtEnd "Test that every suffix of the sample is found at the end." 0 to: stringSize do: [:n | |s i t| s := string copyFrom: stringSize - n + 1 to: stringSize. i := string lastIndexOfSubCollection: s startingAt: stringSize. [stringSize + 1 - n = i] assert. t := string copyFrom: i to: i - 1 + n. [t = s] assert].

testOccurrencesOfEmptyCollection "Test that the empty string occurs at the beginning, at the end, and in between every pair of adjacent characters." [(string occurrencesOfSubCollection: '') = (stringSize + 1)] assert.

testOccurrencesOfUniqueParts "Test that unique parts occur as many times as they should." |repeated| repeated := string , string , string. 1 to: stringSize do: [:start | start to: stringSize do: [:finish | |s n| s := string copyFrom: start to: finish. n := string occurrencesOfSubCollection: s. [n = 1] assert. n := repeated occurrencesOfSubCollection: s. [n = 3] assert]].

On Fri, 22 Apr 2022 at 13:57, David T. Lewis <lewis@mail.msen.com> wrote:

...

Hi Richard,

(CC squeak-dev list, replies to the relevant list please)

On Thu, Apr 21, 2022 at 12:07:32AM +1200, Richard O'Keefe wrote:

...

I've just tracked down a nasty little problem porting some code to Pharo. As a result, I have added to the comments in my own versions of these methods. beginsWith: aSequence "Answer true if aSequence is a prefix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' beginsWith: '' + VisualWorks, Dolphin, astc, GNU ST (where the method is called #startsWith:) and VisualAge (where the method is called #wbBeginsWith:) agree than EVERY sequence begins with an empty prefix. - Squeak and Pharo agree that NO sequence begins with an empty sequence. # ST/X chooses compatibility with Squeak, heaving a big unhappy sigh, and adds #startsWith: to have something sensible to use. Now ST/X *thinks* it is compatible with VW, though it isn't, so I wonder if this was a bug that VW fixed and Squeak didn't? astc goes with the majority here. This is also compatible with Haskell, ML, and with StartsWith in C# and startsWith in Java." ^self beginsWith: aSequence ignoringCase: false

endsWith: aSequence "Answer true if aSequence is a suffix of the receiver. This makes sense for all sequences. There is a compatibility issue concerning 'abc' endsWith: ''. + VisualWorks, Dolphin, astc, GNU ST, and VisualAge (where the method is called #wbEndsWith:) agree that EVERY sequence ends with an empty suffix. - Squeak and Pharo agree that NO sequence ends with an empty suffix. # ST/X chooses compatibility with the majority, apparently unaware that this makes #beginsWith: and #endsWith: inconsistent. astc goes with the majority here. This is also compatible with Haskell, ML, C#, and Java." ^self endsWith: aSequence ignoringCase: false

Does anyone have any idea - why Squeak and Pharo are the odd ones out? - why anyone thought making #beginsWith: and #endsWith:, um, "quirky" was a good idea (it's pretty standard in books on the theory of strings to define "x is a prefix of y iff there is a z such that y = x concatenated with z")

The Squeak behavior was introduced in November 1998 by Dan Ingalls in conjunction with some VM improvements that he was doing, and was included in the Squeak2.2 release at that time.

Prior to that update, Squeak did this: 'abc' beginsWith: '' ==> true 'abc' endsWith: '' ==> true

For Squeak2.2 and later it is: 'abc' beginsWith: '' ==> false 'abc' endsWith: '' ==> false

Pharo presumably inherits this from Squeak.

I am attaching Dan's original change set from the early Squeak update stream so you can see the context.

The change set comment says that "endsWith:, beginsWith:, and match have been rewritten to take advantage of this considerably faster method" (referring to a new primitive that Dan had added to the VM in this change set).

...

I was about to try to file a bug report for the first time, then realised that maybe other people don't think this IS a bug.

I don't think it is a bug. I can't think of a case where it makes sense to say that a string of characters "begins with" or "ends with" a string that contains nothing.

In any case, that's the history of the change in Squeak and Pharo as best as I can reconstruct it.

HTH, Dave