TITLE: Computing Everywhere: Indirection AUTHOR: Eugene Wallingford DATE: January 26, 2013 5:52 PM DESC: ----- BODY:

Alice: The hardest word you'll ever be asked to spell is "ichdericious".

Bob: Yikes. Which word?

A few of us have had fun with the quotations in English and Scheme over the last few days, but this idea is bigger than symbols as data values in programs or even words and strings in natural language. They are examples of a key element of computational thinking, indirection, which occurs in real life all the time. A few years ago, my city built a new water park. To account for the influx of young children in the area, the city dropped the speed limit in the vicinity of the pool from 35 MPH to 25 MPH. The speed limit in that area has been 35 MPH for a long time, and many drivers had a hard time adjusting to the change. So the city put up a new traffic sign a hundred yards up the road, to warn drivers of the coming change. It looks like this one:
traffic sign: 40 MPH speed limit ahead
The white image in the middle of this sign is a quoted version of what drivers see down the road, the usual:
traffic sign: 40 MPH speed limit
Now, many people slow down to the new speed limit well in advance, often before reaching even the warning sign. Maybe they are being safe. Then again, maybe they are confusing a sign about a speed limit sign with the speed limit sign itself. If so, they have missed a level of indirection. I won't claim that computer scientists are great drivers, but I will say that we get used to dealing with indirection as a matter of course. A variable holds a value. A pointer holds the address of a location, which holds a value. A URL refers to a web page. The list goes on. Indirection is a fundamental element in the fabric of computation. As computation becomes an integral part of nearly everyone's daily life, there is a lot to be gained by more people understanding the idea of indirection and recognizing opportunities to put it to work to mutual benefit. Over the last few years, Jon Udell has been making a valiant attempt to bring this issue to the attention of computer scientists and non-computer scientists alike. He often starts with the idea of a hyperlink in a web page, or the URL to which it is tied, as a form of computing indirection that everyone already groks. But his goal is to capitalize on this understanding to sneak the communication strategy of pass by reference into people's mental models. As Udell says, most people use hyperlinks every day but don't use them as well as they might, because the distinction between "pass by value" and "pass by reference" is not a part of their usual mental machinery:
The real problem, I think, is that if you're a newspaper editor, or a city official, or a citizen, pass-by-reference just isn't part of your mental toolkit. We teach the principle of indirection to programmers. But until recently there was no obvious need to teach it to everybody else, so we don't.
He has made the community calendar his working example of pass by reference, and his crusade:
In the case of calendar events, you're passing by value when you send copies of your data to event sites in email, or when you log into an events site and recopy data that you've already written down for yourself and published on your own site. You're passing by reference when you publish the URL of your calendar feed and invite people and services to subscribe to your feed at that URL.
"Pass by reference rather than by value" is one of Udell's seven ways to think like the web, his take on how to describe computational thinking in a world of distributed, network media. That essay is a good start on an essential module in any course that wants to prepare people to live in a digital world. Without these skills, how can we hope to make the best use of technology when it involves two levels of indirection, as shared citations and marginalia do? Quotation in Scheme and pass-by-reference are different issue, but they are related in a fundamental way to the concept of indirection. We need to arm more people with this concept than just CS students learning how programming languages work. -----