Better Software - March 2008 - (Page 14)

Code Craft The Other Side of Complexity by Chuck Allison Civilization advances by extending the number of important operations we can perform without thinking. – Alfred North Whitehead Software development has always been an exercise in managing complexity. It has progressed as a discipline as good minds have created abstractions that transform complexity into simplicity. There was a day, for example, when you had to write something like this: ISTOCKPHOTO mov add mov eax, DWORD PTR _x$[ebp] eax, DWORD PTR _y$[ebp] DWORD PTR _z$[ebp], eax instead of this: int z = x + y; Consider the function in listing 1 to reverse a list. ML uses pattern matching to select which branch of a function definition to execute. In the case of reverse, if the argument is the empty list (nil), then nil is returned. Otherwise the first list element is bound to the variable h and the rest of the list to the variable t to set up a recursive call. The point of this example, however, is revealed in the reverse function’s type signature: val reverse = fn : ‘a list -> ‘a list High-level languages encapsulated the complexity of programming at the machine level. Writing logical and quantitative expressions became simple again. Just as a statement is an abstraction of lower-level instructions, a function, or subroutine, is an abstraction of groups of related statements. Subroutines were the first software abstraction to facilitate things we take for granted today, such as refactoring and reusing code. Hence, functions enable the crucial practice of “separation of concerns” at the procedural level, so developers can build functional units independently. Function Abstraction If writing a function constitutes “statement abstraction,” what is “function abstraction”? I see a number of ways of looking at this. One approach is to group related functions into a single, logical unit, like the functions in C’s header or the static methods in Java’s Character class. But does this really represent a substantively higher level of abstraction? Perhaps function overloading is a slightly better example of function abstraction—but only slightly. Instead of coming up with unique function names on your own, the compiler does it for you. About the same time Dennis Ritchie was designing the C language, researchers in Scotland were working on ML, a strongly typed functional language. ML was the first statically typed language to generalize function definitions across types. fun reverse nil = nil | reverse (h::t) = reverse t @ [h]; Listing 1 The token ‘a is a type variable, like a template parameter in C++. The reverse function can process a homogeneous list of elements of any type, as you can see in the sample execution in listing 2. - reverse[1,2,3]; val it = [3,2,1] : int list - reverse[“one”,”two”,”three”]; val it = [“three”,”two”,”one”] : string list Listing 2 Now you know where C++ got the idea for template type parameters. Accommodating a family of related functions in a single definition surely qualifies as function abstraction. Data Abstraction and Beyond You would think that the next logical step would be to group data and related functions together into a coherent unit. It would have been, too, if it hadn’t already been done almost a decade earlier in the form of classes in Simula-67. Simula-67 introduced classes, inheritance, and subtype polymorphism, the type of function abstraction you achieve every time you override a (virtual) method in a subclass. 14 BETTER SOFTWARE MARCH 2008 www.StickyMinds.com http://www.StickyMinds.com

Table of Contents Feed for the Digital Edition of Better Software - March 2008

Better Software - March 2008 Contents Mark Your Calendar Contributors eLightenment Technically Speaking Code Craft Test Connection Management Chronicles Cover Story: Breaking Ground On SOA Software Development Worst Practices Mind the Gap Product Announcements 10 Things You Might Not Know About... The Last Word Ad Index

Better Software - March 2008

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