SKILL for the Skilled: Making Programs Clear and Concise

The SKILL programming language augments Cadence core tool functionality for Virtuoso and Allegro customers. It is also an important development tool for internal Cadence services organizations as well as Cadence product development groups. We see the value, power, flexibility, and elegance of the language as an enabling tool for customizing and enhancing design environments. These capabilities are made possible by the tight integration of the SKILL programming language into the Cadence platform.

This post introduces an upcoming series of articles, SKILL for the Skilled, which will attempt to better enable users to exploit the power and elegance of this fun and interesting programming language.

WARNING: What you are about to read is HIGHLY OPINIONATED!

SKILL Functions: Short and Clear

SKILL programs are usually made up of functions. A function should be short, clear, and express intent. Some programming languages force the programmer to transform the software problem into what the language can express, rather than allowing the programmer to transform the language to fit the problem at hand. The unfortunate result is often that the SKILL program looks like a C program, and is twice to ten times as long as it needs to be. We'll look at this ability to transform the language in upcoming articles.

Example Program

To illustrate the idea that functions should be short and clear, here are two different implementations of the same conceptual function. The confusing one (#1) uses an imperative style. Avoid this style. The second one (#2) uses a functional style. Without sacrificing clarity, it expresses in two lines what the imperative style expresses in nine lines. It is clearer, probably easier to debug, most certainly more efficient computation-wise, and scales better to larger programs.

Implementation #1 -- needlessly confusing

procedure( abs_less_than_100(x)
  prog( (value)
    value = abs(x)
    if( value < 100
      then return(t)
      else return(nil)
    ) ; if
  ) ; prog
) ; procedure

(procedure (abs_less_than_100 x)
  (abs x) < 100)

What were the programmers thinking?

What are the problems of Implementation #1? What was programmer #1's thought process? He was probably trying to think like a Von Neumann machine -- in terms of an arithmetic unit and moving values around between registers until the goal is achieved. Programmer #2 was probably trying to express a mathematical expression.

A more natural way to think

I claim that the mathematical evaluation model is an easier, more natural way for humans to think than trying awkwardly to think like a machine that's moving data between registers. Don't try to make the problem harder than it is.

Aside from the two very different ways of thinking, there are several other issues I have with implementation #1.

• It declares the useless variable: value. There is normally no need to declare a value which is used only once.
• It unnecessarily uses  prog/return. There is no need to use prog/return if you want to return the value that is already in the tail position.
• It unnecessarily uses if/then/else. There is no need to test an expression for TRUE/FALSE, and thereafter produce the same TRUE/FALSE using if/then/else.
• It fills up half the screen space with lines containing only close parentheses. There is no need to put parentheses on separate lines. Rather use an editor which enforces indentation.
• Its only comments are redundant; and worse, they will probably be out of date as soon as someone edits the function and forgets to check all the comments.

The SKILL language is on the one hand easy to learn and easy to use for simple use-once-and-discard scripting tasks. On the other hand the power and flexibility of the language is evident when experienced programmers develop well designed, high quality SKILL based software applications. The elegance of the SKILL language is often underestimated. A well written SKILL program is easy to understand, takes fewer lines of code to implement, and incurs shorter development times than with most other language alternatives, assuming the developers understand the tools they are using.

Until now, it has been somewhat difficult to find insightful articles written about SKILL and how to apply it to day to day problems. I hope this series of articles enables you to get more benefit and enjoyment out of the SKILL programming language.

Jim Newton 

See Also:

• The Reinvigorated Programmer
• Avoid unnecessary clutter
• What is Lisp?