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Informal APL learning session tonight at 18:30 UTC in https://chat.stackexchange.com/rooms/52405/apl . See https://chat.stackexchange.com/transcript/message/41299896 … if you don't have 20 Stack Exchange rep points
Join our live webinar Total Array Ordering on Thursday 22 March 2018 at 16:00 UTC. Jay & Adám introduce a new feature coming up in Dyalog APL version 17.0 that lets you sort anything. https://dyalog.tv/Webinar Following webinar is scheduled for Thursday 19 April at 15:00 UTC.
We define a tradfn called Ctor (it could be called anything, though) and declare it available from the outside (it must be, as you can't be inside yet when you're just creating the instance).
@wizzwizz4 Well, you can always edit the class script, and all the existing instances will update (this is unique to Dyalog APL, I think), but since a constructor only runs at creation time and never in the base class, it will not run if it was added later. However, any new instances created after the edit will run it.
Here we have an uninitialised field (variable) and a monadic constructor which sets the variable upon construction: Try it online!
@wizzwizz4 It was really complicated to get to work. Indeed, base classes are "cached", because i you delete the base class after making an instance, an unaccessible "phantom" base class is kept until the last instance is gone.
Now:
:Class cl3
∇ None
:Access public
:Implements constructor
⎕←'No arguments.'
∇
∇ One x
:Access public
:Implements constructor
⎕←'1 argument:'x
∇
:EndClass
Here is a class with two constructors. APL will call the appropriate one (niladic or monadic): Try it online!
:Class cl4
∇ None
:Access public
:Implements constructor
⎕←'No arguments.'
∇
∇ One x
:Access public
:Implements constructor
⎕←'1 argument:'x
∇
∇ Two(a b)
:Access public
:Implements constructor
⎕←'2 arguments'a b
∇
:EndClass
@EriktheOutgolfer You need to be more precise with what you mean. Do you mean that different things happen when you ⎕NEW depending on the type of the argument?
@wizzwizz4 So the multiple constructors can take care of number of arguments, but you can of course also handle everything in a single fancy constructor.
@wizzwizz4 No, in APL, all arrays are created equal ;-)
:Class cl6
∇ None
:Access public
:Implements constructor
⎕←'No arguments.'
∇
∇ One x
:Access public
:Implements constructor
⎕←'1 argument:'x
∇
∇ Two(a b)
:Access public
:Implements constructor
⎕←'2 arguments'a b
∇
∇ Three(a b c)
:Access public
:Implements constructor
⎕←'3 arguments:'a b c
∇
:EndClass
We have 0–3. What happens when I call this with more than 3?
@wizzwizz4 You probably mean ⊂1 2 3 4 5 6 7 8 and 1 2 3 4 5 6 7 8.
@wizzwizz4 No, the a single named argument indicates that it can take a variable number of args. If you want to refuse construction with non-scalar arg the constructor will have to inspect the arg and throw an error.
OK, enough about constructors. Lets talk about something fun: Destructors.
Sometimes when an instance ceases to exist, you want to do some clean-up. E.g. when the webserver is closed, you want to free ports and write a message to the log, etc.
@wizzwizz4 No, because ceasing to exist doesn't have any arguments. Should be obvious, no? I suppose you could have a method that takes arguments, does something with them, and then kills the self somehow.
@Adám In Python magic methods are named, e.g. __del__. super().__del__ is a reference to the parent's __del__, going up the MRO (method resolution order, a solution to multiple inheritance) to invoke the closest version. Otherwise the parent class destructors aren't implicitly called if overridden.
How do you stop a parent's destructor from running?
For example, if you subclass your webserver so it shares a socket (like SATA's master/slave).
@wizzwizz4 If something is shared, then it it won't go away when the instance dies. And of course, the instance destructor shouldn't just close the socket. Rather it should check whether there are any others that use it first.
@wizzwizz4 Although I've previously pondered why you can't kill an instance by calling its destructor. That way, you could have destructors with arguments… Anyway, I'm only teaching what Dyalog APL currently offers…
@Adám That might not have been a possibility that entered the original developer's mind. But it should be easy enough to replace the socket itself in that scenario so it didn't close when bidden.
@Adám Because then APL would have to deal with null.
@wizzwizz4 Ah, I see what you mean now. But ⎕EXing an instance has the same problem. Calling the destructor with an argument could just keep that argument on storage for later.
@EriktheOutgolfer It sure does make sense, doesn't it? APL gives value errors for things you don't have access to, e.g. trying to call a public method from outside, or an instance method from of base class..
@dzaima I guess that does allow you to have a single clean-up function which you can call early, e.g. to reset the state of the instance when things went wrong or the user wants to start over.
@EriktheOutgolfer Why not? E.g. an application could e.g. close APL (i.e. itself) when a certain object is closed by the user, rather than dumping the user to the session.
@EriktheOutgolfer That's probably because APL must be restarted after patching. Windows sometimes forces a restart when updating critical parts of itself.
@wizzwizz4 Constructors doesn't make sense. How can the instance override the base constructor before it has been instantiated? If you don't want that constructor, you're using the wrong base class.
@wizzwizz4 Also, there are two ways to override. If the derived class has an identically named method, it overrides the base class method for the derived class only, but if the identically named method is :Access Public Override, then even if code in the base class calls that method, it is redirected to the derived class one.
@wizzwizz4 You can put such common methods in a namespace, and then write :Include MyNamespace. This allows calling that namespace's methods as if they were in the base class. This way, multiple classes can share a set of methods, as if they had multiple inheritance.
Thank you for participating!
… but feel free to hang around and ask questions. Now and any time.
@Pavel Sorry we didn't get around to that this week. However, it does fit better into my planned subject for next week.
@wizzwizz4 Sorry then. I'm not understanding why you would want to include methods from one base class in another. You could just have a base-base class which implements the shared methods for all the classes that need them, no?
@wizzwizz4 The TIO wrapper uses ⎕FIX, yes. But that's an unusual interface to APL. In fact, Header+Code+Footer gets wrapped in :Namespace, :EndNamespace and then that file gets ⎕FIXed, and then the members of that anonymous namespace get copied into #.
@wizzwizz4 Ah, I see. Well, as I said, you can call public methods across from classes, but you'd have to use explicit references to the target class. You could possibly also explicitly copy the members from one class to another. I've not tried doing that, but hang on…
@dzaima Yeah, I think it eventually falls back to a numeric solver if the function is invertible but the internal rules don't suffice to give an exact result.
@wizzwizz4 What? They're just… called. Although if you can capture the "value" of a non-niladic function by using its name without giving it any argument(s).
@wizzwizz4 No, it is copying all visible identifiers. But a class can see itself, so when it looks at the symbol table inside Cl1 it also sees Cl1. But it is easy to code around that.
Now we have a list of names, we call the anonymous function {…} on each ¨ name.
⍵,'←##.Cl1.',⍵ simply concatenates the name (⍵ is right arg) with an assignment arrow and a reference to the name inside Cl1.
Finally, ⍎ evaluates that as an APL expression. Assignments done inside ⍎ are not local to the dfn (the function in braces), so they instead populate the current namespace, i.e. Cl2, which is what we wanted.
@wizzwizz4 Yeah. But a better (though more labour intensive) way to do this, would be to get the representation of each function in the target class with ⎕NR and then establish it here with ⎕FX. There is a difference though: This way will actually create new functions here, while the assignment method will create references.
@wizzwizz4 ^ is a safe method though. But of course, since you want to access (and run, I suppose) code from a 3rd party class, you must trust them not to include malicious code.
@wizzwizz4 It isn't a simple catch-all assignment. For each member, you'd have to find out what type it is. Variables would need copying using assignment to the same name over here. Functions and Operators would need ⎕FX ⎕NR, scripted objects would need ⎕FIX ⎕SRC and unscripted objects would need name ⎕NS ref.
