In Scala, what is an “early initializer”?

问题:

In Martin Odersky's recent post about levels of programmer ability in Scala, in the Expert library designer section, he includes the term "early initializers".

These are not mentioned in Programming in Scala. What are they?

回答1:

Early initializers are part of the constructor of a subclass that is intended to run before its superclass. For example:

abstract class X {
    val name: String
    val size = name.size
}

class Y extends {
    val name = "class Y"
} with X

If the code was written instead as

class Z extends X {
    val name = "class Z"
}

then a null pointer exception would occur when Z got initialized, because size is initialized before name in the normal ordering of initialization (superclass before class).

回答2:

As far as I can tell, the motivation (as given in the link above) is:

"Naturally when a val is overridden, it is not initialized more than once. So though x2 in the above example is seemingly defined at every point, this is not the case: an overridden val will appear to be null during the construction of superclasses, as will an abstract val."

I don't see why this is natural at all. It is completely possible that the r.h.s. of an assignment might have a side effect. Note that such code structure is completely impossible in either C++ or Java (and I will guess Smalltalk, although I can't speak for that language). In fact you have to make such dual assignments implicit...ticilpmi...EXplicit in those languages via constructors. In the light of the r.h.s. side effect uncertainty, it really doesn't seem like much of a motivation at all: the ability to sidestep superclass side effects (thereby voiding superclass invariants) via ASSIGNMENT? Ick!

Are there other "killer" motivations for allowing such unsafe code structure? Object-oriented languages have done without such a mechanism for about 40 years (30-odd years, if you count from the creation of the language), why include it now?

It...just...seems...dangerous.

回答3:

On second thought, a year layer...

This is just cake. Literally.

Not an early ANYTHING. Just cake (mixins).

Cake is a term/pattern coined by The Grand Pooh-bah himself, one that employs Scala's trait system, which is halfway between a class and an interface. It is far better than Java's decoration pattern.

The so-called "interface" is merely an unnamed base class, and what used to be the base class is acting as a trait (which I frankly did not know could be done). It is unclear to me if a "with'd" class can take arguments (traits can't), will try it and report back.

This question and its answer has stepped into one of Scala's coolest features. Read up on it and be in awe.