2.10 A Simple Web Server

In the preceding section, we built the core logic for event-driven GUIs. In this section, we finally extend the core to a real application. No one would actually write a commercial web server in gawk, but it is instructive to see that it is feasible in principle.

The application is ELIZA, the famous program by Joseph Weizenbaum that mimics the behavior of a professional psychotherapist when talking to you. Weizenbaum would certainly object to this description, but this is part of the legend around ELIZA. Take the site-independent core logic and append the following code:

function SetUpServer() {
  TopHeader = \
    "<HTML><title>An HTTP-based System with GAWK</title>\
    <HEAD><META HTTP-EQUIV=\"Content-Type\"\
    CONTENT=\"text/html; charset=iso-8859-1\"></HEAD>\
    <BODY BGCOLOR=\"#ffffff\" TEXT=\"#000000\"\
    LINK=\"#0000ff\" VLINK=\"#0000ff\"\
    ALINK=\"#0000ff\"> <A NAME=\"top\">"
  TopDoc    = "\
   <h2>Please choose one of the following actions:</h2>\
   <A HREF=" MyPrefix "/AboutServer>About this server</A>\
   <A HREF=" MyPrefix "/AboutELIZA>About Eliza</A></LI>\
   <A HREF=" MyPrefix \
      "/StartELIZA>Start talking to Eliza</A></LI></UL>"
  TopFooter = "</BODY></HTML>"

SetUpServer() is similar to the previous example, except for calling another function, SetUpEliza(). This approach can be used to implement other kinds of servers. The only changes needed to do so are hidden in the functions SetUpServer() and HandleGET(). Perhaps it might be necessary to implement other HTTP methods. The igawk program that comes with gawk may be useful for this process.

When extending this example to a complete application, the first thing to do is to implement the function SetUpServer() to initialize the HTML pages and some variables. These initializations determine the way your HTML pages look (colors, titles, menu items, etc.).

The function HandleGET() is a nested case selection that decides which page the user wants to see next. Each nesting level refers to a menu level of the GUI. Each case implements a certain action of the menu. At the deepest level of case selection, the handler essentially knows what the user wants and stores the answer into the variable that holds the HTML page contents:

function HandleGET() {
  # A real HTTP server would treat some parts of the URI as a file name.
  # We take parts of the URI as menu choices and go on accordingly.
  if (MENU[2] == "AboutServer") {
    Document    = "This is not a CGI script.\
      This is an httpd, an HTML file, and a CGI script all \
      in one GAWK script. It needs no separate www-server, \
      no installation, and no root privileges.\
      <p>To run it, do this:</p><ul>\
      <li> start this script with \"gawk -f httpserver.awk\",</li>\
      <li> and on the same host let your www browser open location\
      </ul>\<p>\ Details of HTTP come from:</p><ul>\
            <li>Hethmon:  Illustrated Guide to HTTP</p>\
            <li>RFC 2068</li></ul><p>JK 14.9.1997</p>"
  } else if (MENU[2] == "AboutELIZA") {
    Document    = "This is an implementation of the famous ELIZA\
        program by Joseph Weizenbaum. It is written in GAWK and\
        uses an HTML GUI."
  } else if (MENU[2] == "StartELIZA") {
    gsub(/\+/, " ", GETARG["YouSay"])
    # Here we also have to substitute coded special characters
    Document    = "<form method=GET>" \
      "<h3>" ElizaSays(GETARG["YouSay"]) "</h3>\
      <p><input type=text name=YouSay value=\"\" size=60>\
      <br><input type=submit value=\"Tell her about it\"></p></form>"

Now we are down to the heart of ELIZA, so you can see how it works. Initially the user does not say anything; then ELIZA resets its money counter and asks the user to tell what comes to mind open-heartedly. The subsequent answers are converted to uppercase characters and stored for later comparison. ELIZA presents the bill when being confronted with a sentence that contains the phrase “shut up.” Otherwise, it looks for keywords in the sentence, conjugates the rest of the sentence, remembers the keyword for later use, and finally selects an answer from the set of possible answers:

function ElizaSays(YouSay) {
  if (YouSay == "") {
    cost = 0
  } else {
    q = toupper(YouSay)
    gsub("'", "", q)
    if (q == qold) {
    } else {
      if (index(q, "SHUT UP") > 0) {
        answer = "WELL, PLEASE PAY YOUR BILL. ITS EXACTLY ... $"\
      } else {
        qold = q
        w = "-"                 # no keyword recognized yet
        for (i in k) {          # search for keywords
          if (index(q, i) > 0) {
            w = i
        if (w == "-") {         # no keyword, take old subject
          w    = wold
          subj = subjold
        } else {                # find subject
          subj = substr(q, index(q, w) + length(w)+1)
          wold = w
          subjold = subj        #  remember keyword and subject
        for (i in conj)
           gsub(i, conj[i], q)   # conjugation
        # from all answers to this keyword, select one randomly
        answer = r[indices[int(split(k[w], indices) * rand()) + 1]]
        # insert subject into answer
        gsub("_", subj, answer)
  cost += length(answer) # for later payment : 1 cent per character
  return answer

In the long but simple function SetUpEliza(), you can see tables for conjugation, keywords, and answers.11 The associative array k contains indices into the array of answers r. To choose an answer, ELIZA just picks an index randomly:

function SetUpEliza() {
  wold = "-"
  subjold = " "

  # table for conjugation
  conj[" ARE "     ] = " AM "
  conj["WERE "     ] = "WAS "
  conj[" YOU "     ] = " I "
  conj["YOUR "     ] = "MY "
  conj[" IVE "     ] =\
  conj[" I HAVE "  ] = " YOU HAVE "
  conj[" YOUVE "   ] =\
  conj[" YOU HAVE "] = " I HAVE "
  conj[" IM "      ] =\
  conj[" I AM "    ] = " YOU ARE "
  conj[" YOURE "   ] =\
  conj[" YOU ARE " ] = " I AM "

  # table of all answers
  # table for looking up answers that
  # fit to a certain keyword
  k["CAN YOU"]      = "1 2 3"
  k["CAN I"]        = "4 5"
  k["YOU ARE"]      =\
  k["YOURE"]        = "6 7 8 9"

Some interesting remarks and details (including the original source code of ELIZA) are found on Mark Humphrys’s home page How my program passed the Turing Test. Wikipedia provides much background information about ELIZA, including the original design of the software and its early implementations.



The version shown here is abbreviated. The full version comes with the gawk distribution.