In short they come from to different conceptualizations that, in some languages where double dispatch is not natively supported, lead to the visitor pattern as a way to concatenate two (or more) single dispatch in order to have a multi-dispatch surrogate. In long The idea of multiple dispatch is – essentially – allow a call … Read more
That definition of an Actor actually seems a little restrictive. It certainly doesn’t handle Erlang-style actors (or I believe Scala-style actors). In my experience, an actor is something that: Sends and receives messages (each actor has a mailbox) Shares no mutable memory with other actors Is scheduled based on the whims of the runtime. An … Read more
Those are entirely valid reasons to violate DRY. I should add a third: performance. It’s rarely a big deal, but it can make a difference, and abstraction can risk slowing things down. Actually, I’ll add a fourth: wasting time and potentially introducing new bugs by changing two (or more) parts of a codebase that might … Read more
We used design patterns in the 80’s, we just didn’t know they were design patterns.
A good plugin architecture is difficult to achieve from scratch, but offers its own rewards. It makes the software flexible and simple to maintain by localising complexity. The foremost skill it requires is the ability to write loosely coupled code. This demands a very firm grasp of polymorphism, Demeter’s Law and the related Hollywood principle. … Read more
monostate and singleton are two faces of the same medal (global state): monostate forces a behaviour (only one value along all class instances) singleton forces a structural constraint (only one instance) singleton usage is not transparent i.e.: Singleton singleton = Singleton.getInstance(); monostate usage is transparent i.e.: MonoState m1 = new MonoState(); MonoState m2 = new … Read more
Yes, singletons are bad. They are bad because all they do for you is combine two properties, each of which is bad about 95% of the time. (Which would mean that on average, singletons are bad 99.75% of the time ;)) A singleton, as defined by the GoF, is a data structure which: Grants global … Read more
The Haskell community is split on this issue. John Hughes reports that he finds it easier to teach monad transformers than to teach monads, and that his students do better with a “transformers first” approach. The GHC developers generally avoid monad transformers, preferring to roll up their own monads which integrate all the features they … Read more
Essentially you create an interface and then a concrete implementation of that interface that wraps the classes and methods of Log4net directly. Additional logging systems can be wrapped by creating more concrete classes which wrap other classes and methods of those systems. Finally use a factory to create instances of your wrappers based on a … Read more
In Effective Java, 2nd edition, Joshua Bloch certainly recommends the former. There are a few reasons I can remember, and doubtless some I can’t: You can give the method a meaningful name. If you’ve got two ways of constructing an instance both of which take an int, but have different meanings for that int, using … Read more