I have a swiftlint warning that bothers me.
warning: Nesting Violation: Types should be nested at most 1 level deep (nesting)
However, the nesting of structs is an established programming technique, and quite a few people advocate it.
Edit:
Indeed #vadian points out the Swift language guide's rule:To nest a type within another type, write its definition within the outer braces of the type it supports. Types can be nested to as many levels as are required.
I am aware it clashes with the use of generics, and that Xcode may become unbearably slow. It actually was (through measuring the slowest compilation spots) the reason why I started looking at this nesting rule.
What is the reason for the lint rule, and what is the good practice in that respect? Please point out the technical reasons, rather than purely opinion-based advice.
Microsoft actually has a page about nested types, and when they are appropriate. While it is not targeted at Swift, it does have some interesting trans-language thoughtbits.
After much searching, all I've found is #jpsim's remark that "the idea behind the nesting rule is to avoid complex interfaces".
Therefore, apart from the compiler issues outlined in my question, which will eventually subside, there seems to not be any technical reason for this rule.
Related
Before you relieve the itching in your fingertips, I already understand:
how and when to use the try keyword
the differences between the try, try?, and try! keywords
What I want to understand is what the use of the unadorned try keyword buys me (and you and all of us) over and above merely quieting a compiler diagnostic. We're already inside the scope of a do, and clearly the compiler knows to demand a try, and I can't (yet) see how there might be some ambiguity about where the try needs to land. So why can't the compiler quietly do the right thing without the explicit appearance of the keyword?
There's been a fair amount of discussion (below) about the possibility that the language is trying to enforce readability for humans. I guess we'd need the input from one of the Swift language designers to determine whether that's true. And even if we had that it would be debatable whether it's wise and/or has been a success. So let's put that aside for the moment. Does the existence of the un-adorned try keyword solve some problem other than enforcing readability for humans?
After a long, productive discussion (linked elsewhere on this page)…
In short, the answer is no, there isn't a purpose other than enforcing readability, but it turns out the readability win is more significant than I had realized.
The try keyword should be seen as akin to (though not the equivalent of) a combination of if and goto. Although try doesn't direct the compiler to do anything it could not have inferred it should do, no one would argue that an if or a goto should be invisible. This makes try a little weird for folks coming from other languages — but not unreasonably so.
It may be difficult for Objective C programmers to grasp this because they are accustomed to assuming almost anything they do may raise an exception. Of course, Objective C exceptions are very different from Swift errors, but knowing this consciously is different from metabolizing it and knowing it unconsciously.
As well, if your intuition immediately tells you that as a matter of style in most cases there should probably be only one failable operation inside a do clause, it may be difficult to see what value a try adds.
In Swift, collections are implicitly implemented with copy on write behavior; However, we don't get it for free in our custom types.
My main question is:
Regardless of how to achieve it, is it a good idea to do for our custom types? Why/Why not?
Moreover:
According to this answer, even the built-in types (but not collections) provided from the Swift standard library do not implement it which could be an indication that we don't have to do it. Even so, is there any advantage of doing it?
You do not have to do it, but it can be a worthwhile optimization if you have the resources and need to do so. Ask yourself the following questions:
Is my datatype copied often (i.e. applicability)?
Is it easy enough to implement CoW in reasonably time (i.e. viability)?
Does my application benefit from these optimizations (i.e. return of investment)?
Probably, in most applications it is not necessary and the users will not notice the difference. In some specific cases it might be applicable, but be critical. Remember:
Premature performance optimization is the root of all evil ~ Donald Knuth
I have a swiftlint warning that bothers me.
warning: Nesting Violation: Types should be nested at most 1 level deep (nesting)
However, the nesting of structs is an established programming technique, and quite a few people advocate it.
Edit:
Indeed #vadian points out the Swift language guide's rule:To nest a type within another type, write its definition within the outer braces of the type it supports. Types can be nested to as many levels as are required.
I am aware it clashes with the use of generics, and that Xcode may become unbearably slow. It actually was (through measuring the slowest compilation spots) the reason why I started looking at this nesting rule.
What is the reason for the lint rule, and what is the good practice in that respect? Please point out the technical reasons, rather than purely opinion-based advice.
Microsoft actually has a page about nested types, and when they are appropriate. While it is not targeted at Swift, it does have some interesting trans-language thoughtbits.
After much searching, all I've found is #jpsim's remark that "the idea behind the nesting rule is to avoid complex interfaces".
Therefore, apart from the compiler issues outlined in my question, which will eventually subside, there seems to not be any technical reason for this rule.
In comparing these two options for defining an instance property:
var networkManager = NetworkManager.sharedInstance()
var lazy networkManager = NetworkManager.sharedInstance()
Both:
Can evaluate a block to get the value
Can be declared inline (not a block, like above)
Lazy:
Can refer to self
Is not calculated until needed
If you don't use it, it is never calculated
Non-lazy:
No benefits whatsoever
It appears that there is no benefit to ever use a non-lazy variable. So why does the language allow the programmer to make this inferior choice?
(I am NOT asking about the difference between var and let à la Are Swift constants lazy by default?)
One reason might be that lazyness is not well-suited for situations where you want control when the evaluation happens. this is relevant in cases where the work being done in the assignment has side effects.
Although this pertains to closure, this blog post by stuart sierra explains this idea very well, and I think it applies equally in any language.
As others already said, there are several critical scenarios where you want the initialization of the properties to be deterministic.
This is an example (among many others) related to game development.
Often the instances of classes representing items in a game scene/level, are created before the level does begin.
Initialisation can be a time expensive task (load stuff from persistent storage, allocate memory, prepare the instances...) and doing this part before the player does begin playing the level does avoid CPU overhead.
This is critical because a CPU overhead in the middle of a level could cause a drop in the frame rate which is a nightmare for the user experience.
FYI. My feeling is that Swift wants to become more like a functional language and would like lazy instantiation in more places.
My early assessment of Swift has held up pretty well over time (well, the "not functional" part. I didn't anticipate how much Swift would favor methods over functions in later versions). Swift is not a functional language and does not intend to be one. This has come up often in WWDC talks, on the forums, on Twitter, and in conversations with the Swift team. Originally all maps and filters were lazy. Swift removed that because of the problems it caused. Probably the best talk on that subject is "Building Better Apps with Value Types in Swift". As they say:
We like mutation. We think it's valuable. We think it's easy to use when done correctly.
You don't get much more "non-functional" than that. Swift also embraces immutable data. But functional programming is about pure functions over immutable data, and that's not Swift.
(Of course there are plenty of non-lazy functional languages. Lazy and functional are orthogonal concepts. Haskell just happened to embrace both.)
To the question at hand, though:
I've found the lazy attribute rarely useful in real-world Swift (I'm being generous; I have never encountered a case where I kept it in the code). It doesn't offer anything like the laziness you get in Haskell. It isn't thread safe, so that's a nightmare. It forces you into reference types (or forces your structs to be mutable), so that can be annoying. If I heard they were pulling it from the language and we just had to roll our own, that'd be fine with me. (I'm tempted to write a proposal to do just that.) It implements a specific memo pattern that can occasionally be handy, but often isn't the one you want. So it's a very good thing that it isn't the default.
As you likely know, global variables and class variables are lazy by default, and I think that tends to work out pretty well since there are so many fewer of them, there's a much better chance they won't be accessed in practice, and that laziness is thread safe (which has a cost, but since they're so much rarer, the cost is much lower).
If you have an expensive object (in terms of, takes long to create) you would like to decide and control when it is created. One could argue that the lazy variable should be the default though. Maybe it has historical reasons. Lazy properties in ObjC resulted in a lot boilerplate code.
With the growth of dynamically typed languages, as they give us more flexibility, there is the very likely probability that people will write programs that go beyond what the specification allows.
My thinking was influenced by this question, when I read the answer by bobince:
A question about JavaScript's slice and splice methods
The basic thought is that splice, in Javascript, is specified to be used in only certain situations, but, it can be used in others, and there is nothing that the language can do to stop it, as the language is designed to be extremely flexible.
Unless someone reads through the specification, and decides to adhere to it, I am fairly certain that there are many such violations occuring.
Is this a problem, or a natural extension of writing such flexible languages? Or should we expect tools like JSLint to help be the specification police?
I liked one answer in this question, that the implementation of python is the specification. I am curious if that is actually closer to the truth for these types of languages, that basically, if the language allows you to do something then it is in the specification.
Is there a Python language specification?
UPDATE:
After reading a couple of comments, I thought I would check the splice method in the spec and this is what I found, at the bottom of pg 104, http://www.mozilla.org/js/language/E262-3.pdf, so it appears that I can use splice on the array of children without violating the spec. I just don't want people to get bogged down in my example, but hopefully to consider the question.
The splice function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the splice function
can be applied successfully to a host object is implementation-dependent.
UPDATE 2:
I am not interested in this being about javascript, but language flexibility and specs. For example, I expect that the Java spec specifies you can't put code into an interface, but using AspectJ I do that frequently. This is probably a violation, but the writers didn't predict AOP and the tool was flexible enough to be bent for this use, just as the JVM is also flexible enough for Scala and Clojure.
Whether a language is statically or dynamically typed is really a tiny part of the issue here: a statically typed one may make it marginally easier for code to enforce its specs, but marginally is the key word here. Only "design by contract" -- a language letting you explicitly state preconditions, postconditions and invariants, and enforcing them -- can help ward you against users of your libraries empirically discovering what exactly the library will let them get away with, and taking advantage of those discoveries to go beyond your design intentions (possibly constraining your future freedom in changing the design or its implementation). And "design by contract" is not supported in mainstream languages -- Eiffel is the closest to that, and few would call it "mainstream" nowadays -- presumably because its costs (mostly, inevitably, at runtime) don't appear to be justified by its advantages. "Argument x must be a prime number", "method A must have been previously called before method B can be called", "method C cannot be called any more once method D has been called", and so on -- the typical kinds of constraints you'd like to state (and have enforced implicitly, without having to spend substantial programming time and energy checking for them yourself) just don't lend themselves well to be framed in the context of what little a statically typed language's compiler can enforce.
I think that this sort of flexibility is an advantage as long as your methods are designed around well defined interfaces rather than some artificial external "type" metadata. Most of the array functions only expect an object with a length property. The fact that they can all be applied generically to lots of different kinds of objects is a boon for code reuse.
The goal of any high level language design should be to reduce the amount of code that needs to be written in order to get stuff done- without harming readability too much. The more code that has to be written, the more bugs get introduced. Restrictive type systems can be, (if not well designed), a pervasive lie at worst, a premature optimisation at best. I don't think overly restrictive type systems aid in writing correct programs. The reason being that the type is merely an assertion, not necessarily based on evidence.
By contrast, the array methods examine their input values to determine whether they have what they need to perform their function. This is duck typing, and I believe that this is more scientific and "correct", and it results in more reusable code, which is what you want. You don't want a method rejecting your inputs because they don't have their papers in order. That's communism.
I do not think your question really has much to do with dynamic vs. static typing. Really, I can see two cases: on one hand, there are things like Duff's device that martin clayton mentioned; that usage is extremely surprising the first time you see it, but it is explicitly allowed by the semantics of the language. If there is a standard, that kind of idiom may appear in later editions of the standard as a specific example. There is nothing wrong with these; in fact, they can (unless overused) be a great productivity boost.
The other case is that of programming to the implementation. Such a case would be an actual abuse, coming from either ignorance of a standard, or lack of a standard, or having a single implementation, or multiple implementations that have varying semantics. The problem is that code written in this way is at best non-portable between implementations and at worst limits the future development of the language, for fear that adding an optimization or feature would break a major application.
It seems to me that the original question is a bit of a non-sequitor. If the specification explicitly allows a particular behavior (as MUST, MAY, SHALL or SHOULD) then anything compiler/interpreter that allows/implements the behavior is, by definition, compliant with the language. This would seem to be the situation proposed by the OP in the comments section - the JavaScript specification supposedly* says that the function in question MAY be used in different situations, and thus it is explicitly allowed.
If, on the other hand, a compiler/interpreter implements or allows behavior that is expressly forbidden by a specification, then the compiler/interpreter is, by definition, operating outside the specification.
There is yet a third scenario, and an associated, well defined, term for those situations where the specification does not define a behavior: undefined. If the specification does not actually specify a behavior given a particular situation, then the behavior is undefined, and may be handled either intentionally or unintentionally by the compiler/interpreter. It is then the responsibility of the developer to realize that the behavior is not part of the specification, and, should s/he choose to leverage the behavior, the developer's application is thereby dependent upon the particular implementation. The interpreter/compiler providing that implementation is under no obligation to maintain the officially undefined behavior beyond backwards compatibility and whatever commitments the producer may make. Furthermore, a later iteration of the language specification may define the previously undefined behavior, making the compiler/interpreter either (a) non-compliant with the new iteration, or (b) come out with a new patch/version to become compliant, thereby breaking older versions.
* "supposedly" because I have not seen the spec, myself. I go by the statements made, above.