Skip to main content

Too Many Moving Parts

A common, if somewhat informal, observation about a large code base is that there are "too many moving parts" in it. In my experience, this is especially true for large Java systems but is probably universally true.


What do we mean by ‘too many moving parts’?


Simply put, there is always a significant semantic gap between a programming language and the program. The larger this gap, the more that has to be expressed in the language, as opposed to simply using it.


For example, consider the problem of traversing a recursive tree structure. In Java, we can iterate over an Iterable; structure using a loop, for example, to count elements:



int count = 0;
for(E el:tree)
{
  count++;
}

If the Tree class did not implement Iterable we would be forced to construct an explicit iterator (or worse, write a recursive one-off function):



int count = 0;
for(Iterator<E> it=tree.iterator(); it.hasNext();)
{
  E el = it.next();
  count++;
}

This version illustrates what happens when a programming language does not quite meet us halfway in our programming task. There is a lot of extra clutter (managing the iterator) that makes it hard to see what is really going on.


In this case, Java's for notation makes it significantly easier to see the program. However, there are many cases where this is not true. For example, you cannot use a similar technique for reading files, searching or removing elements from a tree, etc. etc.


Language Extensions


One way of reducing clutter is to permit the programmer to extend the language. Of course, the designers of Java set their face against this — there is no macro facility in Java — for a reasonable if misguided reason: to prevent programmers lying to each other.


The Star language does permit language extensions to be introduced by the programmer. This has the effect of encapsulating not only data abstractions but also control abstractions.


For example, to count the elements of a tree in Star, we can do:



var count:=0;
for E in tree do
  count := count+1;

The program depends on the programmer implementing the type contract for _search and a macro expansion rule:



# for ?Ptn in ?Exp do ?Act ==>
  __search(Exp,procedure(X){ if X matches Ptn then Act})

No apologies for the macro definition itself, but the effect is that the language has been lifted into one that fits the requirements more closely. This, in turn, reduces the semantic gap between the language as used by the programmer and the application.



Of course, there is quite a bit more to reducing clutter than macro definitions. However, it should be an important goal of language design to ensure that programmers can express themselves with minimum extraneous concepts.

Labels:

Popular posts from this blog

Comments Should be Meaningless

This is something of a counterintuitive idea: Comments should be meaningless What, I hear you ask, are you talking about? Comments should communicate to the reader! At least that is the received conventional wisdom handed does over the last few centuries (decades at least). Well, certainly, if you are programming in Assembler, or C, then yes, comments should convey meaning because the programming language cannot So, conversely, as a comment on the programming language itself, anytime the programmer feels the imperative to write a meaningful comment it is because the language is not able to convey the intent of the programmer. I have already noticed that I write far fewer comments in my Java programs than in my C programs.  That is because Java is able to capture more of my meaning and comments would be superfluous. So, if a language were able to capture all of my intentions, I would never need to write a comment. Hence the title of this blog.
Read more

Action at a distance

We are currently writing our first draft of the SOA Reference Architecture. Everyone is very busy doing their bit. My current section is on the Real World Effect of using a service. The RA is really an abstract architecture: we are not focusing on things like SOAP, or any of the other 60+ Web service specifications out there. We are trying to get at the essence of makes SOA special and how it can be made to work. It is a pretty basic aspect of services that we are trying to get something to happen: buy a book, get the weather forecast whatever. In other words: its action at a distance. I am communicating with you in the hope that we can get some mutual benefit. This already distinguishes SOA from the Web, whose basic abstraction is to acquire a representation of a resource will be rendered locally for human consumption. Actions are not inherently about representations, they are about changing the world - one book at a time. Action itself is a very difficult concept to get hold of. It ...
Read more

Organizing principles for services

One of the questions that comes up from time to time is how to define your services. This has come up for me in two independent fora: within the OASIS Service Oriented Architecture work and in the context of human provided services, for example at Genietown . In the work on the SOA Reference Model we decided that "services are the mechanism by which needs and capabilities are brought together"; i.e., its about needs and capabilities to satisfy those needs, and the access mechanism. However, this still begs the question somewhat. In the domain of human services, where the services are things like "building a home", "walking the dog", "taking care of my elderly parents"; it gets even fuzzier. Sometimes a service seems to organized around the person offering the service, for example, an architect, or a doctor. Sometimes the service is organized around a particular kind of product, such as doors or skylights. At other times, the service is organize...
Read more