TITLE: Inverting the Relationship Between Programs and Literals
AUTHOR: Eugene Wallingford
DATE: December 24, 2013 11:35 AM
DESC: 
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BODY:
<A HREF="http://erights.org/elang/grammar/quasi-overview.html">
   This chapter</A>
on quasi-literals in the E language tells the story of
<A HREF="http://www.scottkim.com/">
   Scott Kim</A>
teaching the author that Apple's
<A HREF="http://en.wikipedia.org/wiki/HyperCard">
   HyperCard</A>
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?

<BLOCKQUOTE><EM>
 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.
</EM></BLOCKQUOTE>

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 <TT>eval</TT>'ed inside of literal text.  Of
course, the resulting text was then passed on the <TT>eval</TT>
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:
<PRE>
    &lt;body&gt;
    &lt;?php
      echo $_SERVER['HTTP_USER_AGENT'];
    ?&gt;
    &lt;/body&gt;
</PRE>

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
<A HREF="http://mustache.github.io/">
   Mustache</A>-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
<A HREF="http://hypercard.org/">
   an ardent following</A>.
Check out
<A HREF="http://zhurnaly.com/cgi-bin/wiki/R.I.P._HyperCard">
   this brief eulogy</A>,
which rhapsodizes on "the mystically-enchanting mantra" at the
end of the application's About box: "<EM>A day of acquaintance, /
And then the longer span of custom. / But first -- / The hour of
astonishment.</EM>")
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