Thursday, August 23, 2007

Static Analysis as Grammar Checking

When I explain what I intend to study to friends who don't program, I say something like the text below. I'm still working out the kinks in the analogy, but this is my starting point:

When we write, we sometimes make mistakes:

I am hunry when I wake up.
So we invented spelling checkers to catch this simple mistake. We did the same thing for computer languages a long time ago, to catch simple mistakes like spelling errors. And, just as we then invented grammar checkers to catch sentences like:
I am hungrily when I wake up.
that spelling checkers wouldn't catch, we also invented more sophisticated methods to catch more sophisticated mistakes in computer programs.

But grammar rules can sometimes restrict how expressive our sentences are if we wish to speak informally, or to be funny or poetic, or to quote someone famous:

You've got a friend in Pennsylvania.
What price justice?
Hand me a hypodeemic nerdle.
For computer programs, we try to characterize useful patterns (like "rhetorical question" or "implicit subject in imperative" in English) and build sets of rules called type systems. These sets of rules are designed to allow these patterns without allowing incorrect constructions. Type systems require all incorrect constructions to be corrected by the author before a program is run.

At the same time, we can build more permissive tools that look for particular patterns that are likely to be errors, like repeated words or a lowercase letter following a question mark. Anything flagged by the tool as possibly erroneous can be inspected by the author and corrected if necessary.

The goal of all of this work (called static analysis) is to catch bugs in software before it is shipped and bothers users or blows up aerospace equipment.

My goal is to build new static analysis tools to help prevent rocket explosions and Mac commercials.

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