string is rarely useful as a function argument

[Timon Gehr]

On 01/01/2012 02:34 AM, Chad J wrote:
> On 12/31/2011 02:02 PM, Timon Gehr wrote:
>> On 12/31/2011 07:22 PM, Chad J wrote:
>>> On 12/30/2011 02:55 PM, Timon Gehr wrote:
>>>> On 12/30/2011 08:33 PM, Joshua Reusch wrote:
>>>>> Am 29.12.2011 19:36, schrieb Andrei Alexandrescu:
>>>>>> On 12/29/11 12:28 PM, Don wrote:
>>>>>>> On 28.12.2011 20:00, Andrei Alexandrescu wrote:
>>>>>>>> Oh, one more thing - one good thing that could come out of this
>>>>>>>> thread
>>>>>>>> is abolition (through however slow a deprecation path) of
>>>>>>>> s.length and
>>>>>>>> s[i] for narrow strings. Requiring s.rep.length instead of s.length
>>>>>>>> and
>>>>>>>> s.rep[i] instead of s[i] would improve the quality of narrow strings
>>>>>>>> tremendously. Also, s.rep[i] should return ubyte/ushort, not
>>>>>>>> char/wchar.
>>>>>>>> Then, people would access the decoding routines on the needed
>>>>>>>> occasions,
>>>>>>>> or would consciously use the representation.
>>>>>>>>
>>>>>>>> Yum.
>>>>>>>
>>>>>>>
>>>>>>> If I understand this correctly, most others don't. Effectively, .rep
>>>>>>> just means, "I know what I'm doing", and there's no change to
>>>>>>> existing
>>>>>>> semantics, purely a syntax change.
>>>>>>
>>>>>> Exactly!
>>>>>>
>>>>>>> If you change s[i] into s.rep[i], it does the same thing as now.
>>>>>>> There's
>>>>>>> no loss of functionality -- it's just stops you from accidentally
>>>>>>> doing
>>>>>>> the wrong thing. Like .ptr for getting the address of an array.
>>>>>>> Typically all the ".rep" everywhere would get annoying, so you would
>>>>>>> write:
>>>>>>> ubyte [] u = s.rep;
>>>>>>> and use u from then on.
>>>>>>>
>>>>>>> I don't like the name 'rep'. Maybe 'raw' or 'utf'?
>>>>>>> Apart from that, I think this would be perfect.
>>>>>>
>>>>>> Yes, I mean "rep" as a short for "representation" but upon first sight
>>>>>> the connection is tenuous. "raw" sounds great.
>>>>>>
>>>>>> Now I'm twice sorry this will not happen...
>>>>>>
>>>>>
>>>>> Maybe it could happen if we
>>>>> 1. make dstring the default strings type --
>>>>
>>>> Inefficient.
>>>>
>>>
>>> But correct (enough).
>>>
>>>>> code units and characters would be the same
>>>>
>>>> Wrong.
>>>>
>>>
>>> *sigh*, FINE.  Code units and /code points/ would be the same.
>>
>> Relax.
>>
>
> I'll do one better and ultra relax:
> http://www.youtube.com/watch?v=jimQoWXzc0Q
> ;)
>
>>>
>>>>> or 2. forward string.length to std.utf.count and opIndex to
>>>>> std.utf.toUTFindex
>>>>
>>>> Inconsistent and inefficient (it blows up the algorithmic complexity).
>>>>
>>>
>>> Inconsistent?  How?
>>
>> int[]
>> bool[]
>> float[]
>> char[]
>>
>
> I'll refer to another limb of this thread when foobar mentioned a mental
> model of strings as strings of letters.  Now, given annoying corner
> cases, we probably can't get strings of /letters/, but I'd at least like
> to make it as far as code points.  That seems very doable.  I mention
> this because I find that forwarding string.length and opIndex would be
> much more consistent with this mental model of strings as strings of
> unicode code points, which, IMO, is more important than it being binary
> consistent with the other things.  I'd much rather have char[] behave
> more like an array of code points than an array of bytes.  I don't need
> an array of bytes.  That's ubyte[]; I have that already.
>

char[] is not an array of bytes: it is an array of UTF-8 code units.

>>>
>>> Inefficiency is a lot easier to deal with than incorrect.  If something
>>> is inefficient, then in the right places I will NOTICE.  If something is
>>> incorrect, it can hide for years until that one person (or country, in
>>> this case) with a different usage pattern than the others uncovers it.
>>>
>>>>>
>>>>> so programmers could use the slices/indexing/length (no lazyness
>>>>> problems), and if they really want codeunits use .raw/.rep (or better
>>>>> .utf8/16/32 with std.string.representation(std.utf.toUTF8/16/32)
>>>>>
>>>>
>>>> Anyone who intends to write efficient string processing code needs this.
>>>> Anyone who does not want to write string processing code will not need
>>>> to index into a string -- standard library functions will suffice.
>>>>
>>>
>>> What about people who want to write correct string processing code AND
>>> want to use this handy slicing feature?  Because I totally want both of
>>> these.  Slicing is super useful for script-like coding.
>>>
>>
>> Except that the proposal would make slicing strings go away.
>>
>
> Yeah, Andrei's proposal says that.  But I'm speaking of Joshua's:
>
>>>>> so programmers could use the slices/indexing/length ...
>
> I kind-of like either, but I'd prefer Joshua's suggestion.
>
>>>>> But generally I liked the idea of just having an alias for strings...
>>>>
>>>> Me too. I think the way we have it now is optimal. The only reason we
>>>> are discussing this is because of fear that uneducated users will write
>>>> code that does not take into account Unicode characters above code point
>>>> 0x80. But what is the worst thing that can happen?
>>>>
>>>> 1. They don't notice. Then it is not a problem, because they are
>>>> obviously only using ASCII characters and it is perfectly reasonable to
>>>> assume that code units and characters are the same thing.
>>>>
>>>
>>> How do you know they are only working with ASCII?  They might be /now/.
>>>    But what if someone else uses the program a couple years later when the
>>> original author is no longer maintaining that chunk of code?
>>
>> Then they obviously need to fix the code, because the requirements have
>> changed. Most of it will already work correctly though, because UTF-8
>> extends ASCII in a natural way.
>>
>
> Or, you know, we could design the language a little differently and make
> this become mostly a non-problem.  That would be cool.
>

It is imo already mostly a non-problem, but YMMV:

void main(){
     string s = readln();
     int nest = 0;
     foreach(x;s){ // iterates by code unit
         if(x=='(') nest++;
         else if(x==')' && --nest<0) goto unbalanced;
     }
     if(!nest){
         writeln("balanced parentheses");
         return;
     }
unbalanced:
     writeln("unbalanced parentheses");
}

That code is UTF aware, even though it does not explicitly deal with 
UTF. I'd claim it is like this most of the time.


>>>
>>>> 2. They get screwed up string output, look for the reason, patch up
>>>> their code with some functions from std.utf and will never make the same
>>>> mistakes again.
>>>>
>>>
>>> Except they don't.  Because there are a lot of programmers that will
>>> never put in non-ascii strings to begin with.  But that has nothing to
>>> do with whether or not the /users/ or /maintainers/ of that code will
>>> put non-ascii strings in.  This could make some messes.
>>>
>>>>
>>>> I have *never* seen an user in D.learn complain about it. They might
>>>> have been some I missed, but it is certainly not a prevalent problem.
>>>> Also, just because an user can type .rep does not mean he understands
>>>> Unicode: He is able to make just the same mistakes as before, even more
>>>> so, as the array he is getting back has the _wrong element type_.
>>>>
>>>
>>> You know, here in America (Amurica?) we don't know that other countries
>>> exist.  I think there is a large population of programmers here that
>>> don't even know how to enter non-latin characters, much less would think
>>> to include such characters in their test cases.  These programmers won't
>>> necessarily be found on the internet much, but they will be found in
>>> cubicles all around, doing their 9-to-5 and writing mediocre code that
>>> the rest of us have to put up with.  Their code will pass peer review
>>> (their peers are also from America) and continue working just fine until
>>> someone from one of those confusing other places decides to type in the
>>> characters they feel comfortable typing in.  No, there will not be
>>> /tests/ for code points greater than 0x80, because there is no one
>>> around to write those.  I'd feel a little better if D herds people into
>>> writing correct code to begin with, because they won't otherwise.
>>>
>>
>> There is no way to 'herd people into writing correct code' and UTF-8 is
>> quite easy to deal with.
>>
>
> Probably not.  I played fast and loose with this a lot in my early D
> code.  Then this same conversation happened like ~3 years ago on this
> newsgroup.  Then I learned more about unicode and had a bit of a bitter
> taste regarding char[] and how it handled indexing.  I thought I could
> just index char[]s willy nilly.  But no, I can't.  And the compiler
> won't tell me.  It just silently does what I don't want.
>

How often do you actually need to get, for example, the 10th character 
of a string? I think it is a very uncommon operation. If the indexing is 
just part of an iteration that looks once at each char and handles some 
ASCII characters in certain ways, there is no potential correctness 
problem. As soon as code talks about non-ascii characters, it has to be 
UTF aware anyway.

> Maybe unicode is easy, but we sure as hell aren't born with it, and the
> language doesn't give beginners ANY red flags about this.
>
> I find myself pretty fortified against this issue due to having known
> about it before anything unpleasant happened, but I don't like the idea
> of others having to learn the hard way.
>

Hm, well. The first thing I looked up when I learned D supports Unicode 
is how Unicode/UTF work in detail. After that, the semantics of char[] 
were very clear to me.

>>> ...
>>>
>>> There's another issue at play here too: efficiency vs correctness as a
>>> default.
>>>
>>> Here's the tradeoff --
>>>
>>> Option A:
>>> char[i] returns the i'th byte of the string as a (char) type.
>>> Consequences:
>>> (1) Code is efficient and INcorrect.
>>
>> Do you have an example of impactful incorrect code resulting from those
>> semantics?
>>
>
> Nope.  Sorry.  I learned about it before it had a chance to bite me.
> But this is only because I frequent(ed) the newsgroup and had a good
> throw on my dice roll.
>

I might be wrong, but I somewhat have the impression we might be chasing 
phantoms here. I have so far never seen a bug in real world code caused 
by inadvertent misuse of D string indexing or slicing.

>>> (2) It requires extra effort to write correct code.
>>> (3) Detecting the incorrect code may take years, as these errors can
>>> hide easily.
>>
>> None of those is a direct consequence of char[i] returning char. They
>> are the consequence of at least 3 things:
>>
>> 1. char[] is an array of char
>> 2. immutable(char)[] is the default string type
>> 3. the programmer does not know about 1. and/or 2.
>>
>> I say, 1. is inevitable. You say 3. is inevitable. If we are both right,
>> then 2. is the culprit.
>>
>
> I can get behind this.
>
> Honestly I'd like the default string type to be intelligent and optimize
> itself into whichever UTF-N encoding is optimal for content I throw into
> it.  Maybe this means it should lazily expand itself to the narrowest
> character type that maintains a 1-to-1 ratio between code units and code
> points so that indexing/slicing remain O(1), or maybe it's a bag of
> disparate encodings, or maybe someone can think of a better strategy.
> Just make it /reasonably/ fast and help me with correctness as much as
> possible.  If I need more performance or more unicode pedantics, I'll do
> my homework then and only then.
>
> Of course this is probably never going to happen I'm afraid.  Even the
> problem of making such a (probably) struct work at compile time in
> templates as if it were a native type... agh, headaches.
>
>>>
>>> Option B:
>>> char[i] returns the i'th codepoint of the string as a (dchar) type.
>>> Consequences:
>>> (1) Code is INefficient and correct.
>>
>> It is awfully optimistic to assume the code will be correct.
>>
>>> (2) It requires extra effort to write efficient code.
>>> (3) Detecting the inefficient code happens in minutes.  It is VERY
>>> noticable when your program runs too slowly.
>>>
>>
>> Except when in testing only small inputs are used and only 2 years later
>> maintainers throw your program at a larger problem instance and wonder
>> why it does not terminate. Or your program is DOS'd. Polynomial blowup
>> in runtime can be as large a problem as a correctness bug in practice
>> just fine.
>>
>
> I see what you mean there.  I'm still not entirely happy with it though.
>   I don't think these are reasonable requirements.  It sounds like forced
> premature optimization to me.
>

It is using a better algorithm that performs faster by a linear factor. 
I would be very leery of something that looks like a constant time array 
indexing operation take linear time. I think premature optimization is 
about writing near-optimal hard-to-debug and maintain code that only 
gains some constant factors in parts of the code that are not 
performance critical.

> I have found myself in a number of places in different problem domains
> where optimality-is-correctness.  Make it too slow and the program isn't
> worth writing.  I can't imagine doing this for workloads I can't test on
> or anticipate though: I'd have to operate like NASA and make things 10x
> more expensive than they need to be.
>
> Correctness, on the other hand, can be easily (relatively speaking)
> obtained by only allowing the user to input data you can handle and then
> making sure the program can handle it as promised.  Test, test, test, etc.
>
>>>
>>> This is how I see it.
>>>
>>> And I really like my correct code.  If it's too slow, and I'll /know/
>>> when it's too slow, then I'll profile->tweak->profile->etc until the
>>> slowness goes away.  I'm totally digging option B.
>>
>> Those kinds of inefficiencies build up and make the whole program run
>> sluggish, and it will possibly be to late when you notice.
>>
>
> I get the feeling that the typical divide-and-conquer profiling strategy
> will find the more expensive operations /at least/ most of the time.
> Unfortunately, I have only experience to speak from on this matter.
>

Yes, what I meant is, that if the inefficiencies are spread out more or 
less uniformly, then fixing it all up might seem to be too much work and 
too much risk.

>> Option B is not even on the table. This thread is about a breaking
>> interface change and special casing T[] for T in {char, wchar}.
>>
>>
>
> Yeah, I know.  I'm refering to what Joshua wrote, because I like option
> B.  Even if it's academic, I'll say I like it anyways, if only for the
> sake of argument.

OK.