TITLE: Inverting the Relationship Between Programs and Literals AUTHOR: Eugene Wallingford DATE: December 24, 2013 11:35 AM DESC: ----- BODY: This chapter on quasi-literals in the E language tells the story of Scott Kim teaching the author that Apple's HyperCard was "powerful in a way most of had not seen before". This unexpected power led many people to misunderstand its true importance. Most programs are written as text, sequences of characters. In this model, a literal is a special form of embedded text. When the cursor is inside the quotes of a literal string, we are "effectively no longer in a program editor, but in a nested text editor". Kim calls this a 'pun': Instead of writing text to be evaluated by another program, we are creating output directly. What if we turn things inside out and embed our program in literal text?
Hypercard is normally conceived of as primarily a visual application builder with an embedded silly programming language (Hypertalk). Think instead of a whole Hypercard stack as a mostly literal program. In addition to numbers and strings, the literals you can directly edit include bitmaps, forms, buttons, and menus. You can literally assemble most things, but where you need dynamic behavior, there's a hole in the literal filled in by a Hypertalk script. The program editor for this script is experienced as being nested inside the direct manipulation user interface editor.
There is a hole in the literal text, where a program goes, instead of a hole in the program, where literal text goes. HyperTalk must surely have seemed strange to most programmers in 1987. Lisp programmers had long used macros and so knew the power of nesting code to be eval'ed inside of literal text. Of course, the resulting text was then passed on the eval to be treated again as program! The inside-out idea of HyperCard is alive today in the form of languages such as PHP, which embed code in HTML text:
      echo $_SERVER['HTTP_USER_AGENT'];
This is a different way to think about programming, one perhaps suitable for bringing experts in some domains toward the idea of writing code gradually from documents in their area of expertise. I sometimes have the students in my compiler course implement a processor for a simple Mustache-like template language as an early warm-up homework assignment. I do not usually require them to go as far as Turing-complete embedded code, but they create a framework that makes it possible. I think I'll look for ways to bring more of this idea into the next offering of our more general course on programming languages. (HyperCard really was more than many people realized at the time. The people who got it became Big Fans, and the program still has an ardent following. Check out this brief eulogy, which rhapsodizes on "the mystically-enchanting mantra" at the end of the application's About box: "A day of acquaintance, / And then the longer span of custom. / But first -- / The hour of astonishment.") -----