TITLE: Real-Life Examples of Quotation in Scheme AUTHOR: Eugene Wallingford DATE: January 24, 2013 4:32 PM DESC: ----- BODY: The new semester is fully underway, and I'm already enjoying Programming Languages. My Tuesday session this week felt like a hodgepodge of topics, including Scheme definitions and conditionals, and didn't inspire my students much. Today's session on pairs and lists seemed to go much more smoothly, at least from my side of the classroom. One thing that has been different the first two weeks this time around has been several questions about the quote character in Scheme, which is shorthand for the special form quote. The purpose of the quote is to tell the interpreter to take its argument literally. When the argument is a list, say, '(* 2 3), quotation prevents the interpreter from evaluating the list as a Scheme procedure call. When the argument is a symbol, say, 'a, the quote lets the interpreter know not to treat the a as an identifier, looking up the value bound to that name in the current environment. Instead, it is treated as the literal symbol a. Most of our students have not yet worked in languages where symbols are first-class data values, so this idea takes some getting used to. In the course of talking about quotation with them, I decided to relate this idea to an example of quotation from real life. The first thing that came to mind at that instant was the distinction between these two sentences:
Lincoln was disappointing.In the former, Lincoln is a name to be evaluated. Depending on the context, it could refer to the 16th president of the United States, the capital of Nebraska, or some other object in the world. (The sentence doesn't have to be true, of course!) In the latter, quoting Lincoln makes it a title. I intended for this "literal" reference to the word Lincoln to evoke the current feature film of that name. Almost immediately I began to second-guess my example. The quoted Lincoln is still a name for something -- a film, or a boo, or some such -- and so still needs to be "dereferenced" to retrieve the object signified. It's just that we treat titles differently than other names. So it's close to what I wanted to convey, but it could mislead students in a dangerous way. The canonical real-world example of quotation is to quote a word so that we treat the utterance as the word itself. Consider:
"Lincoln" was disappointing.
Creativity is overused.In the former, creativity is a name to be evaluated. It signifies an abstract concept, a bundle of ideas revolving around creation, originality, art, and ingenuity. We might say creativity is overused in a context where people should be following the rules but are instead blazing their own trails. In the latter, the quoted creativity signifies the word itself, taken literally. We might say "creativity" is overused to suggest an author improve a piece of writing by choosing a near-synonym such as "cleverness" or "originality", or by rephrasing a sentence so that the abstract concept is recast as the verb in an active statement. This example stays more faithful to the use of quote in Scheme, where an expression is taken literally, with no evaluation of of any kind needed. I like giving examples of how programming concepts exist in other parts of our lives and world. Even when they are not perfect matches, they can sometimes help a student's mind click on the idea as it works in a programming language or style. I like it better when I use better examples! -----
"Creativity" is overused.