mg,
> On 15 Aug 2018, at 11:15 PM, mg <[email protected]> wrote:
>
> But in addition to this problem, you have the problem that for at least every
> commutative* function/operator (plus, times, etc), you have to make sure that
>
> notNullVariable.commutativeFunctionOrOperator(null)
>
> must now return null, since
If I really wanted to completely get rid of NPE, I would have to.
Nevertheless,
(a) it would be a vain effort; any called method could e.g., throw NPE
explicitly if it decides to. No remedy for that!
(b) it is a normal way of things, just consider
foo?.bar(baz)
where baz happens to be null, foo does not, and foo.bar() does throw NPE upon a
null argument. “Safe navigaiton” not so safe in fact! :) It is ugly, but
inevitable.
Luckily, in practice, it is not a problem, for it does not happen often.
> Since you cannot know if an arbitrary function is commutative, the function
> itself would need to implement the correct behavior of returning null in that
> case. My impression is that it seems exceedingly hard to bend a null-semantic
> language into a nil-direction, if it has not been built that way to begin
> with.
Actually this applies to ObjC either: although “nil.whatever(whichever_value)“
is guaranteed by the runtime to be an empty nil-returning operation,
“foo.whatever(bar)” for a non-nil foo definitely can throw upon a nil bar (it
would not be NPE, which does not exist there, but it might be e.g., an
InvalidArgumentException).
No way to get rid of that — and no problem in practice either.
> Apart from that I do not see the advantage you claim right now, and to me it
> is a bug that in dynamic Groovy null.each { ... } simply does nothing (in
> static Groovy this throws a NPE as expected), but I reserve judgement until I
> see your larger side by side example :-)
I do not know the implementation and behaviour myself, but conceptually, I do
believe that
- null.each { ... } should NPE
- null?.each { ... } should silently do nothing
quite regardless of static/dynamic. As always, I might be overlooking something
of importance.
All the best.,
OC
>
> -------- Ursprüngliche Nachricht --------
> Von: "ocs@ocs" <[email protected]>
> Datum: 15.08.18 19:53 (GMT+00:00)
> An: [email protected]
> Betreff: Re: suggestion: ImplicitSafeNavigation annotation
>
> Eric,
>
>> On 15 Aug 2018, at 4:14 PM, Milles, Eric (TR Technology & Ops)
>> <[email protected] <mailto:[email protected]>>
>> wrote:
>> Does your requested change amount to setting the "isSafe" condition on all
>> PropertyExpression instances in a block?
>
> That has been the original idea ages ago, when I have started playing with
> that.
>
> It did not prove quite feasible: you cannot “safe” operators this way, there
> are those ugly quirks of super?.whatever etc. I have mentioned before...
>
>> That is the AST equivalent of adding the '?' to each '.'
>
> Quitte, only some of '.'s do not take '?' gently and sport a weird results,
> from an insane (though understandable) runtime behaviour like
> “null?.is(null)" sports up through exceptions compile-time to (at least to me
> not understandable) Verity Error caused by super?.anything.
>
> Thanks and all the best,
> OC
>
>>
>> From: ocs@ocs <[email protected] <mailto:[email protected]>>
>> Sent: Wednesday, August 15, 2018 7:55 AM
>> To: [email protected] <mailto:[email protected]>
>> Cc: [email protected] <mailto:[email protected]>
>> Subject: Re: suggestion: ImplicitSafeNavigation annotation
>>
>> H2,
>>
>>> On 15 Aug 2018, at 9:47 AM, [email protected] <mailto:[email protected]> wrote:
>>>
>>> OCS, your reference to Objective-C is interesting, as is your expectation
>>> for your code to encounter null values often.
>>
>> In my experience, a null is (should be, at the very least) a first-class
>> citizen, value which simply means “there is no object there”. In normal code
>> it happens all the time, e.g., if you check a map with a key which happens
>> not to be there, or if you use find with a condition which happens not to
>> match any of the items in the list, etc.
>>
>>> I may be old school, but I think the semantics of null values is
>>> interesting, as is the semantics of safe navigation.
>>
>> Actually, all this NPE stuff (and its consequences) is pretty new-school :)
>>
>> The aforementioned Objective C (of eighties!) has a couple of very simple
>> rules:
>> - wherever an expression returns an object, it may return a nil;
>> - whenever a message[*] is sent to a nil, it is a very quick and efficient
>> empty operation, whose return value is again nil.
>>
>> [*] in ObjC, you use an object exclusively[**] through sending messages: you
>> send a message without any argument for a property getter, you send a
>> message with one argument for a property setter, you send a message with an
>> arbitrary number of arguments for a method call, etc.
>> [**] but for the access to instance variables (more or less “fields”), which
>> is sort-of similar to plain-C structs, and happens from the declaring class
>> code only, usually is limited to the property accessor methods and never
>> used outside of them.
>>
>> Whilst I do completely appreciate that tastes and experiences do differ,
>> well, myself, in thirty-odd years of using this paradigm daily and after an
>> uncounted zillions source lines in projects some of which have 25-odd-year
>> lifespan, were made for NeXTSTEP ages ago and are maintained for macOS of
>> today, I am yet to find any drawback of this approach. Cases when a bug has
>> been caused by unintended and uncaught sending a message to a nil are
>> extremely rare (myself I can recall about three of such cases in all those
>> years), whilst the advantages for code readability, conciseness, and
>> robustness are tremendous.
>>
>>> Safe navigation addresses the matter of nested if statements, perfectly in
>>> my opinion.
>>
>> Absolutely. Nevertheless, there's absolutely no point why it should be
>> limited to methods — compare e.g., the following case (very simple and thus
>> a bit artificial to keep concise):
>>
>> ===
>> interface TreeNode {
>> TreeNode createChildIfPossible(String name); // if possible, creates and
>> returns a new named child; null if not possible
>> }
>> void makeSubtreeIfPossible(TreeNode tn) { // here, we do not care whether
>> successful; just want to make as big a subtree as possible
>> def foo=tn?.createChildIfPossible('foo')
>> foo?.createChildIfPossible('bar')?.createChildIfPossible('baz')
>>
>> foo?.createChildIfPossible('bax')?.createChildIfPossible('baz')?.createChildIfPossible('last')
>> }
>> ===
>>
>> Thanks to ?. we have dodged the necessity of super-ugly ifs, but still, the
>> result is sort of at the ugly javaish side. Can we get groovier? Well of
>> course we can:
>>
>> ===
>> class TreeNode {
>> def leftShift(object) { this.createChildIfPossible(object) }
>> }
>> void makeSubtreeIfPossible(TreeNode tn) {
>> def foo=tn?<<'foo'
>> foo?<<'bar'?<<'baz'
>> foo?<<'bax'?<<'baz'?<<'last'
>> }
>> ===
>>
>> Oh, oops! We can't do that, for we do not have “safe navigation” for
>> operators, and thus, if we want to use the << for adding a child, we would
>> have to get back to ugly Javaish ifs, which I would not dare to show lest
>> some reader might get sick :)
>>
>>> What's more, it is explicit in terms of where it applies - you can combine
>>> safe navigation with "unsafe navigation", allowing NPE's to be thrown and
>>> nulls to propagate where appropriate.
>>
>> Agreed, and where one needs just an occasional null-propagation, it's
>> perfect (or, as the example above shows, would be, if it could be used with
>> all the operators instead of just with the method call, property access and
>> indexing ones).
>>
>> Nevertheless, there are cases where one needs the null-propagation not just
>> occasionally, but very often (if not exclusively) in a piece of code — do
>> not all the ?'s in the examples above look ugly? And even if you like them,
>> they very definitely make the code highly fragile: it is very easy and quite
>> probable to simply forget one or two of them, getting thus one unwanted,
>> uncaught and potentially disastrous NPE (essentially in random based on the
>> data, so testing might not help to find&fix the culprit). You would rather
>> have to add a try/catch harness, which in this case creates a boilerplate
>> code. We should do without boilerplate in Groovy, in my opinion!
>>
>> On the other hand, this would be clear, concise, completely
>> intention-revealing, and completely robust:
>>
>> ===
>> @ImplicitSafeNavigation(true) void makeSubtreeIfPossible(TreeNode tn) {
>> def foo=tn<<'foo'
>> foo<<'bar'<<'baz'
>> foo<<'bax'<<'baz'<<'last'
>> }
>> ===
>>
>>> Unless the same explicit control can be exerted over arithmetic / other
>>> expressions, I think those two concepts cannot be compared.
>>
>> Seems to me it's just one concept, not two of them.
>>
>> It has been available from the very beginning for two kinds of expressions:
>> a method call and a property getter.
>>
>> Lately, it has been extended to another kind of expression, namely, the
>> indexing.
>>
>> It would be only consistent and reasonable to extend the thing to all
>> expressions without an exception.
>>
>> Regardless whether that happens or not, still, it is only one and the same
>> concept; we are not debating any other one, but just an extent of its
>> availability. Especially in Groovy, where... oh, see please below :)
>>
>>> I also think any assumption about the semantics of null values are
>>> problematic. In particular, assuming that any null value in an expression
>>> should be propagated is not obvious to me.
>>
>> To me, it seems very obvious, completely natural and quite intuitive —
>> especially in Groovy where (unlike many other languages) the “expression
>> operators” are essentially just a convenience syntax sugar for plain method
>> calls (i.e., “foo<<bar” is nothing but a convenience shorthand for
>> equivalent but ugly “foo.leftShift(bar)”, “foo+bar” for “foo.plus(bar)”, and
>> so forth).
>>
>> Thanks and all the best,
>> OC
>>
>>> Den 2018-08-15 04:18, skrev ocs@ocs:
>>>> mg,
>>>>> On 15 Aug 2018, at 3:26 AM, mg <[email protected]
>>>>> <mailto:[email protected]>> wrote:
>>>>> Fair enough (I am typing this on my smartphone on vacation, so keep
>>>>> samples small; also (your) more complex code samples are really hard
>>>>> to read in my mail reader). It still seems to be a big paradigm
>>>>> change
>>>> I might be missing something of importance here, but I can't see any
>>>> paradigm change; not even the slightest shift.
>>>> The only change suggested is that one could — in the extent of one
>>>> needs that, which would self-evidently differ for different people —
>>>> decide whether the “safe” behaviour is explicitly triggered by
>>>> using the question-mark syntax, or whether it is implicit.
>>>>> since regular Java/Groovy programs typically have very little null
>>>>> values
>>>> The very existence of ?. and ?[] suggests it is not quite the case —
>>>> otherwise, nobody would ever bother designing and implementing them.
>>>>> so am not convinced this is worth the effort (and as Jochen pointed
>>>>> out, there will still be cases where null will just be converted to
>>>>> "null").
>>>> Are there? Given my limited knowledge, I know of none such.
>>>> “null?.plus('foo')” yields a null, and so — for a consistency
>>>> sake — very definitely should also “null?+'foo'” and
>>>> “@ImplicitSafeNavigation ... null+foo”, had they existed.
>>>>> What I would suggest instead is considering to introduce nil,
>>>>> sql_null, empty, ... as type agnostic constants in addition to the
>>>>> existing null in Groovy. That way you could use e.g. nil in your
>>>>> code, which by definition exhibits your expected behavior, but it
>>>>> would make the usage more explicit, and one would not need to
>>>>> switch/bend the existing null semantics...
>>>> That's a nice idea; alas, so that it is viable, one would also have to
>>>> be able to set up which kind of null is to be returned from
>>>> expressions like “aMap['unknownkey']“ or “list.find {
>>>> never-matches }” etc.
>>>> Thus, instead of my “@ImplicitSafeNavigation(true)” you would have
>>>> to use something like “@DefaultNullClass(nil)” — and instead of
>>>> “@ImplicitSafeNavigation(false)” you would need something like
>>>> “@DefaultNullClass(null)”.
>>>> Along with that, you would need a way to return “the current default
>>>> null” instead of just null; there would be a real problem with a
>>>> legacy code which returns null (but should return “the current
>>>> default null” instead), and so forth.
>>>> That all said, it definitely is an interesting idea worth checking;
>>>> myself, though, I do fear it would quickly lead to a real mess (unlike
>>>> my suggestion, which is considerably less flexible, but at the same
>>>> moment, very simple and highly intuitive).
>>>> Thanks and all the best,
>>>> OC
>>>>> -------- Ursprüngliche Nachricht --------
>>>>> Von: "ocs@ocs" <[email protected] <mailto:[email protected]>>
>>>>> Datum: 15.08.18 00:53 (GMT+00:00)
>>>>> An: [email protected] <mailto:[email protected]>
>>>>> Betreff: Re: suggestion: ImplicitSafeNavigation annotation
>>>>> mg,
>>>>>> On 15 Aug 2018, at 1:33 AM, mg <[email protected]
>>>>>> <mailto:[email protected]>> wrote:
>>>>>> That's not how I meant my sample eval helper method to be used :-)
>>>>>> (for brevity I will write neval for eval(true) here)
>>>>>> What I meant was: How easy would it be to get a similar result to
>>>>>> what you want, by wrapping a few key places (e.g. a whole method
>>>>>> body) in your code in neval { ... } ? Evidently that would just
>>>>>> mean that any NPE inside the e.g. method would lead to the whole
>>>>>> method result being null.
>>>>> Which is a serious problem. Rarely you want „a whole method be
>>>>> skipped (and return null) if anything inside of it happens to be
>>>>> null“. What you normally want is the null-propagation, e.g.,
>>>>> def foo=bar.baz[bax]?:default_value;
>>>>> ... other code ...
>>>>> The other code is _always_ performed and _never_ skipped (unless
>>>>> another exception occurs of course); but the null-propagation makes
>>>>> sure that if bar or bar.baz happens to be a null, then default_value
>>>>> is used. And so forth.
>>>>>> To give a simple example:
>>>>>> final x = a?.b?.c?.d
>>>>>> could be written as
>>>>>> final x = neval { a.b.c.d }
>>>>> Precisely. Do please note that even your simple example did not put
>>>>> a whole method body into neval, but just one sole expression
>>>>> instead. Essentially all expressions — often sub-expressions,
>>>>> wherever things like Elvis are used — would have to be embedded in
>>>>> nevals separately. Which is, alas, far from feasible.
>>>>>> Of course the two expressions are not semantically identical,
>>>>>> since neval will transform any NPE inside evaluation of a, b, c,
>>>>>> and d into the result null - but since you say you never want to
>>>>>> see any NPEs...
>>>>> That indeed would not be a problem.
>>>>>> (The performance of neval should be ok, since I do not assume that
>>>>>> you expect your code to actually encounter null values, and
>>>>>> accordingly NPEs, all the time)
>>>>> This one possibly would though: I _do_ expect my code to encounter
>>>>> null values often — with some code, they might well be the normal
>>>>> case with a non-null an exception. That's precisely why I do not
>>>>> want NPEs (but the quick, efficient and convenient null-propagation
>>>>> instead) :)
>>>>> Thanks and all the best,
>>>>> OC
>>>>> -------- Ursprüngliche Nachricht --------
>>>>> Von: "ocs@ocs" <[email protected] <mailto:[email protected]>>
>>>>> Datum: 14.08.18 23:14 (GMT+00:00)
>>>>> An: [email protected] <mailto:[email protected]>
>>>>> Betreff: Re: suggestion: ImplicitSafeNavigation annotation
>>>>> mg,
>>>>> On 14 Aug 2018, at 11:36 PM, mg <[email protected]
>>>>> <mailto:[email protected]>> wrote:
>>>>> I am wondering: In what case does what you are using/suggesting
>>>>> differ significantly from simply catching a NPE that a specific code
>>>>> block throws and letting said block evaluate to null in that case:
>>>>> def eval(bool nullSafeQ, Closure cls) {
>>>>> try {
>>>>> return cls()
>>>>> }
>>>>> catch(NullPointerException e) {
>>>>> if(nullSafeQ) {
>>>>> return null
>>>>> }
>>>>> throw e
>>>>> }
>>>>> }
>>>>> Conceptually, not in the slightest.
>>>>> In practice, there's a world of difference.
>>>>> For one, it would be terrible far as the code cleanness, fragility
>>>>> and readability are concerned — even worse than those ubiquitous
>>>>> question marks:
>>>>> === the code should look, say, like this ===
>>>>> @ImplicitSafeNavigation def foo(bar) {
>>>>> def x=baz(bar.foo)?:bax(bar.foo)
>>>>> x.allResults {
>>>>> def y=baz(it)
>>>>> if (y>1) y+bax(y-1)
>>>>> else y–bax(0)
>>>>> }
>>>>> }
>>>>> === the eval-based equivalent would probably look somewhat like this
>>>>> ===
>>>>> def foo(bar) {
>>>>> def x=eval(true){baz(eval(true){bar.foo})?:bax(bar.foo)}
>>>>> eval(true){
>>>>> x.allResults {
>>>>> def y=eval(true){baz(it)}
>>>>> if (y>1) eval(true){y+bax(y-1)}
>>>>> else eval(true){y–bax(0)}
>>>>> }
>>>>> }
>>>>> }
>>>>> ===
>>>>> and quite frankly I am not even sure whether the usage of eval above
>>>>> is right and whether I did not forget to use it somewhere where it
>>>>> should have been. It would be _ways_ easier with those question
>>>>> marks.
>>>>> Also, with the eval block, there might be a bit of a problem with
>>>>> the type information: I regret to say I do not know whether we can
>>>>> in Groovy declare a method with a block argument in such a way that
>>>>> the return type of the function is automatically recognised by the
>>>>> compiler as the same type as the block return value? (Definitely I
>>>>> don't know how to do that myself; Cédric or Jochen might, though
>>>>> ;))
>>>>> Aside of that, I wonder about the efficiency; although premature
>>>>> optimisation definitely is a bitch, still an exception harness is
>>>>> not cheap if an exception is caught, I understand.
>>>>> (It feels a bit like what you wants is tri-logic/SQL type NULL
>>>>> support in Groovy, not treating Java/Groovy null differently...)
>>>>> In fact what I want is a bit like the Objective-C simple but very
>>>>> efficient and extremely practical nil behaviour, to which I am used
>>>>> to and which suits me immensely.
>>>>> Agreed, the Java world takes a different approach (without even the
>>>>> safe navigation where it originated!); I have tried to embrace that
>>>>> approach a couple of times, and always I have found it seriously
>>>>> lacking.
>>>>> I do not argue that the null-propagating behaviour is always better;
>>>>> on the other hand, I do argue that sometimes and for some people it
>>>>> definitely is better, and that Groovy should support those times and
>>>>> people just as well as it supports the NPE-based approach of Java.
>>>>> Thanks and all the best,
>>>>> OC
>>>>> -------- Ursprüngliche Nachricht --------
>>>>> Von: "ocs@ocs" <[email protected] <mailto:[email protected]>>
>>>>> Datum: 14.08.18 17:46 (GMT+00:00)
>>>>> An: [email protected] <mailto:[email protected]>
>>>>> Betreff: Re: suggestion: ImplicitSafeNavigation annotation
>>>>> Jochen,
>>>>> On 14 Aug 2018, at 6:25 PM, Jochen Theodorou <[email protected]
>>>>> <mailto:[email protected]>>
>>>>> wrote:
>>>>> Am 14.08.2018 um 15:23 schrieb ocs@ocs:
>>>>> H2,
>>>>> However, “a+b” should work as one would expect
>>>>> Absolutely. Me, I very definitely expect that if a happens to be
>>>>> null, the result is null too. (With b null it depends on the details
>>>>> of a.plus implementation.)
>>>> the counter example is null plus String though
>>>> Not for me. In my world, if I am adding a string to a non-existent
>>>> object, I very much do expect the result is still a non-existent
>>>> object. Precisely the same as if I has been trying to turn it to
>>>> lowercase or to count its character or anything.
>>>> Whilst I definitely do not suggest forcing this POV to others, to me,
>>>> it seems perfectly reasonable and 100 per cent intuitive.
>>>> Besides, it actually (and expectably) does work so, if I use the
>>>> method-syntax to be able to use safe navigation:
>>>> ===
>>>> 254 /TMP> <q.groovy
>>>> String s=null
>>>> println "Should be null: ${s?.plus('foo')}"
>>>> 255 /TMP> /usr/local/groovy-2.4.15/bin/groovy q
>>>> WARNING: An illegal reflective access operation has occurred
>>>> ... ...
>>>> Should be null: null
>>>> 256 /TMP>
>>>> ===
>>>> which is perfectly right. Similarly, a hypothetical “null?+'foo'”
>>>> or “@ImplicitSafeNavigation ... null+foo” should return null as
>>>> well, to keep consistent.
>>>> (Incidentally, do you — or anyone else — happen to know how to get
>>>> rid of those pesky warnings?)
>>>> Thanks and all the best,
>>>> OC
>
