Note that this question and similar ones have been asked before, such as in Forward References - why does this code compile?, but I found the answers to still leave some questions open, so I'm having another go at this issue.
Within methods and functions, the effect of the val keyword appears to be lexical, i.e.
def foo {
println(bar)
val bar = 42
}
yielding
error: forward reference extends over definition of value bar
However, within classes, the scoping rules of val seem to change:
object Foo {
def foo = bar
println(bar)
val bar = 42
}
Not only does this compile, but also the println in the constructor will yield 0 as its output, while calling foo after the instance is fully constructed will result in the expected value 42.
So it appears to be possible for methods to forward-reference instance values, which will, eventually, be initialised before the method can be called (unless, of course, you're calling it from the constructor), and for statements within the constructor to forward-reference values in the same way, accessing them before they've been initialised, resulting in a silly arbitrary value.
From this, a couple of questions arise:
Why does val use its lexical compile-time effect within constructors?
Given that a constructor is really just a method, this seems rather inconsistent to entirely drop val's compile-time effect, giving it its usual run-time effect only.
Why does val, effectively, lose its effect of declaring an immutable value?
Accessing the value at different times may result in different results. To me, it very much seems like a compiler implementation detail leaking out.
What might legitimate usecases for this look like?
I'm having a hard time coming up with an example that absolutely requires the current semantics of val within constructors and wouldn't easily be implementable with a proper, lexical val, possibly in combination with lazy.
How would one work around this behaviour of val, getting back all the guarantees one is used to from using it within other methods?
One could, presumably, declare all instance vals to be lazy in order to get back to a val being immutable and yielding the same result no matter how they are accessed and to make the compile-time effect as observed within regular methods less relevant, but that seems like quite an awful hack to me for this sort of thing.
Given that this behaviour unlikely to ever change within the actual language, would a compiler plugin be the right place to fix this issue, or is it possible to implement a val-alike keyword with, for someone who just spent an hour debugging an issue caused by this oddity, more sensible semantics within the language?
Only a partial answer:
Given that a constructor is really just a method ...
It isn't.
It doesn't return a result and doesn't declare a return type (or doesn't have a name)
It can't be called again for an object of said class like "foo".new ("bar")
You can't hide it from an derived class
You have to call them with 'new'
Their name is fixed by the name of the class
Ctors look a little like methods from the syntax, they take parameters and have a body, but that's about all.
Why does val, effectively, lose its effect of declaring an immutable value?
It doesn't. You have to take an elementary type which can't be null to get this illusion - with Objects, it looks different:
object Foo {
def foo = bar
println (bar.mkString)
val bar = List(42)
}
// Exiting paste mode, now interpreting.
defined module Foo
scala> val foo=Foo
java.lang.NullPointerException
You can't change a val 2 times, you can't give it a different value than null or 0, you can't change it back, and a different value is only possible for the elementary types. So that's far away from being a variable - it's a - maybe uninitialized - final value.
What might legitimate usecases for this look like?
I guess working in the REPL with interactive feedback. You execute code without an explicit wrapping object or class. To get this instant feedback, it can't be waited until the (implicit) object gets its closing }. Therefore the class/object isn't read in a two-pass fashion where firstly all declarations and initialisations are performed.
How would one work around this behaviour of val, getting back all the guarantees one is used to from using it within other methods?
Don't read attributes in the Ctor, like you don't read attributes in Java, which might get overwritten in subclasses.
update
Similar problems can occur in Java. A direct access to an uninitialized, final attribute is prevented by the compiler, but if you call it via another method:
public class FinalCheck
{
final int foo;
public FinalCheck ()
{
// does not compile:
// variable foo might not have been initialized
// System.out.println (foo);
// Does compile -
bar ();
foo = 42;
System.out.println (foo);
}
public void bar () {
System.out.println (foo);
}
public static void main (String args[])
{
new FinalCheck ();
}
}
... you see two values for foo.
0
42
I don't want to excuse this behaviour, and I agree, that it would be nice, if the compiler could warn consequently - in Java and Scala.
So it appears to be possible for methods to forward-reference instance
values, which will, eventually, be initialised before the method can
be called (unless, of course, you're calling it from the constructor),
and for statements within the constructor to forward-reference values
in the same way, accessing them before they've been initialised,
resulting in a silly arbitrary value.
A constructor is a constructor. You are constructing the object. All of its fields are initialized by JVM (basically, zeroed), and then the constructor fills in whatever fields needs filling in.
Why does val use its lexical compile-time effect within constructors?
Given that a constructor is really just a method, this seems rather
inconsistent to entirely drop val's compile-time effect, giving it its
usual run-time effect only.
I have no idea what you are saying or asking here, but a constructor is not a method.
Why does val, effectively, lose its effect of declaring an immutable value?
Accessing the value at different times may result in different
results. To me, it very much seems like a compiler implementation
detail leaking out.
It doesn't. If you try to modify bar from the constructor, you'll see it is not possible. Accessing the value at different times in the constructor may result in different results, of course.
You are constructing the object: it starts not constructed, and ends constructed. For it not to change it would have to start out with its final value, but how can it do that without someone assigning that value?
Guess who does that? The constructor.
What might legitimate usecases for this look like?
I'm having a hard time coming up with an example that absolutely
requires the current semantics of val within constructors and wouldn't
easily be implementable with a proper, lexical val, possibly in
combination with lazy.
There's no use case for accessing the val before its value has been filled in. It's just impossible to find out whether it has been initialized or not. For example:
class Foo {
println(bar)
val bar = 10
}
Do you think the compiler can guarantee it has not been initialized? Well, then open the REPL, put in the above class, and then this:
class Bar extends { override val bar = 42 } with Foo
new Bar
And see that bar was initialized when printed.
How would one work around this behaviour of val, getting back all the
guarantees one is used to from using it within other methods?
Declare your vals before using them. But note that constuctor is not a method. When you do:
println(bar)
inside a constructor, you are writing:
println(this.bar)
And this, the object of the class you are writing a constructor for, has a bar getter, so it is called.
When you do the same thing on a method where bar is a definition, there's no this with a bar getter.
Related
In D, can I initialize directly on declaration and expect the initializer expressions are part of the constructor?
I came from C# and there this is the case. But with DMD 2.071.0 Im getting other behavior.
class Other { }
class Test { Other nonStaticMember = new Other; }
void test()
{
auto t1 = new Test;
auto t2 = new Test;
// Assert is failing, the two Test instances are
// being initialized to the same reference
// instead of execute the Other constructor twice.
assert(t1.nonStaticMember !is t2.nonStaticMember);
}
If this is the intented behavior it should be documented here: https://dlang.org/spec/class.html right?
This code doesn't do in D what it would do in C#.
In your example, Other is instantiated during compilation.
I.e. Other is instantiated once, during compilation, and is placed in the program's data segment. Then, nonStaticMember's initial value will, by default, point to that instance of Other, for all Test instances.
So, everything is working exactly as designed, even if it may be surprising when coming from other languages.
If this is the intented behavior it should be documented here: https://dlang.org/spec/class.html right?
Perhaps, but note that this behavior is not at all specific to classes. A pointer to any value allocated on the heap, as the initial value of a global or local static variable, will behave the same. Whenever the value of an expression is demanded during compilation (and that includes initializers for global/static variables), D attempts to evaluate it at compile-time. A few years ago, this has been extended to allocating values on the "heap" too, which then end up in the program's initial data segment.
I am trying to call the super constructor from a class using a method. The whole setup looks like this:
class Straight(hand: Hand) extends Combination(Straight.makeHandAceLowIfNeeded(hand), 5)
object Straight {
private def makeHandAceLowIfNeeded(hand: Hand): Hand = {
...
}
}
While this does compile, it has some rather odd runtime behaviour. While debugging, I noticed that the Straight instances have the "hand" property defined twice. Can somebody tell me what is going on, and what the proper way is to call the super constructor with different arguments?
In my use case, I want to call the super constructor with a modified hand in which I replaced a card compared to the original constructor argument.
Debugger screenshot with duplicate field:
.
It's a perfectly fine way to call the superclass constructor. These are two private fields and they don't conflict, though you can rename one of them to avoid confusion during debugging (or if you want to access the superclass' value from the subclass). However, the field should only be generated for a class parameter if it's used outside a constructor, and in your case it doesn't appear to be. Did you simplify the definition of Straight?
I have a function foo which takes another function (say bar) as a parameter. Is there a way to get the function name of bar as a string inside foo?
No. See the difference between methods and functions. Methods aren't passed as parameters under the hood - they are expanded into function objects when being passed to some other method/function. These function objects are instances of anonymous, compiler-generated classes , and have no name (or, at least, being anonymous classes, have some mangled name which you could access using reflection, but probably don't need).
So, when you do:
def foo() {}
def bar(f: () => Unit) {}
bar(foo)
what actually happens in the last call is:
bar(() => foo())
Theoretically, though, you could find the name of the method that the function object you're being passed is wrapping. You could do bytecode introspection to analyze the body of the apply method of the function object f in method bar above, and conclude based on that what the name of the method is. This is both an approximation and an overkill, however.
I've had quite a dig around, and I don't think that there is. toString on the function object just says eg <function1>, and its class is a synthesised class generated by the compiler rather that something with a method object inside it that you might query.
I guess that if you really needed this there would be nothing to stop you implementing function with something that delegated but also knew the name of the thing to which it was delegating.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Why to use empty parentheses in Scala if we can just use no parentheses to define a function which does not need any arguments?
Consider we have a class Foo with a method bar (which does takes no arguments and return the string "bar"). There are two ways to implement bar
The first one is
class Foo {
def bar() = "bar"
}
The second one is
class Foo {
def bar = "bar"
}
While both do basically the same, they need to be called differently, the first one like this
someFoo.bar()
and the second one
someFoo.bar
Why should I use one over the other and what's the fundamental difference?
Defining a method without arguments without parentheses implies that the method is pure (it has no side effects and doesn't depend on the state of program). Such methods cannot be called with parentheses:
class Square(val side: Int) {
def area = side * side
}
s = new Square(10);
s.area //ok
s.area() //compilation error
Calling a method without arguments with parentheses implies that the method has some side effects and a return type of Unit. A method defined with empty parentheses can be called with or without them.
class Foo {
def bar(): Unit = println("bar")
}
f = new Foo();
f.bar; //ok, bad style
f.bar(); // good
Neither of them needs to be called with parantheses. Yet def bar = "bar" is required to be called without parantheses, since the parantheses will be implied to be applied to it's result, thus in this case calling bar() will have the same effect as "bar"().
It's only a matter of convention. In my practice I've seen two:
Standard (used in the standard library and most 3rd party ones). Drop the parantheses when the method produces no side effects. Being "pure", i.e. besides not causing any side effects also not depending on the state, however is not a requirement. According to this convention your second example would be the correct one.
Scalaz. Drop the parantheses whenever there are no arguments to a method, i.e. the method may produce side effects. For example they pimp a method print without parentheses.
Bozhidar presented another convention, but, honestly, it's the first time I've been exposed to it.
I have a function foo which takes another function (say bar) as a parameter. Is there a way to get the function name of bar as a string inside foo?
No. See the difference between methods and functions. Methods aren't passed as parameters under the hood - they are expanded into function objects when being passed to some other method/function. These function objects are instances of anonymous, compiler-generated classes , and have no name (or, at least, being anonymous classes, have some mangled name which you could access using reflection, but probably don't need).
So, when you do:
def foo() {}
def bar(f: () => Unit) {}
bar(foo)
what actually happens in the last call is:
bar(() => foo())
Theoretically, though, you could find the name of the method that the function object you're being passed is wrapping. You could do bytecode introspection to analyze the body of the apply method of the function object f in method bar above, and conclude based on that what the name of the method is. This is both an approximation and an overkill, however.
I've had quite a dig around, and I don't think that there is. toString on the function object just says eg <function1>, and its class is a synthesised class generated by the compiler rather that something with a method object inside it that you might query.
I guess that if you really needed this there would be nothing to stop you implementing function with something that delegated but also knew the name of the thing to which it was delegating.