The Icon Programming Language

Ralph E. Griswold and Gregg M. Townsend

Department of Computer Science, The University of Arizona

Abstract

Introduction

• analysis of text
• representation of structural relationships
• ways of presenting results in an easily understood manner

Background

An Overview of Icon

•   support for strings of characters of arbitrary length
•   high-level facilities for analyzing strings
•   a powerful expression-evaluation mechanism
•   sophisticated data structures for managing complex relationships among data
•   high-level graphics facilities
•   automatic storage management

Expression Evaluation

while line := read() do
   process(line)

"My student is lazier than even the average student."

s1 := "student"
s2 := "My student is lazier than even the average student."

first := find(s1, s2)

every i := find(s1, s2) do
   write(i)

every write(find(s1, s2))

every i := 1 to 10 do
   write(sqrt(i))

"the" | "The" | "THE"

find("the" | "The" | "THE", text)

String Scanning

1. It is typical for many analysis operations to be performed on the same string. Imagine parsing a sentence, for example. The parsing is likely to require many operations on the sentence to identify its components.

2. Many analysis operations occur at a particular place in a string, and the place typically advances as analysis continues. Again, think of parsing a sentence; parsing typically starts at the beginning of the sentence and progresses toward the end as components are identified. Of course, if an initial analysis proves to be incorrect later on, the analysis may go back to an earlier position and look for an alternative.

s ? expr

tab(i), which sets the position to i

move(i), which increments the position by i

text ? {
   while write(move(1)) do
      move(1)
   }

upto(s), which returns the position at which a character of s occurs in the subject

many(s), which returns the position after a sequence of characters of s

text ? {
   while tab(upto(&letters)) do
      write(tab(many(&letters)))
   }

Procedures

procedure genword(text)
   text ? {
      while tab(upto(&letters)) do {
         word := tab(many(&letters))
         suspend word
         }
      }
   fail
end

while line := read() do
   every write(genword(line))

Structures

Lists

parties := ["Socialist", "Democratic", "Republican"]

write(parties[2])

every write(!parties)

put(parties, "Libertarian")

put(parties, "Communist")

word_list := []                           # empty list
while line := read() do
   every put(word_list, genword(line))    # append words

every write(!sort(word_list))

Sets

word_set := set()                        # empty set
while line := read() do
   every insert(word_set, genword(line)) # insert words

every write(!sort(word_set))

word_set := set()
while line := read() do
   every word := genword(line) do
      if not member(word_set, word) then {
      write(word)
      insert(word_set, word)
      }

all_words := word_set1 ++ word_set2

common_words := word_set1 ** word_set2

Tables

word_count := table(0)                     # empty table
while line := read() do
   every word := genword(line) do
      word_count[word] := word_count[word] + 1

procedure main()

   word_count := table(0)                 # empty table

   while line := read() do
      every word := genword(line) do
         word_count[word] := word_count[word] + 1

   result := sort(word_count, 3)

   while write(left(get(result), 15), right(get(result), 8))

end

procedure genword(text)
   text ? {
      while tab(upto(&letters)) do {
         word := tab(many(&letters))
         suspend word
         }
      }
   fail
end

Bells                 1
Future                1
Of                    3
On                    1
To                    2
and                   2
bells                10
chiming               1
how                   1
impells               1
it                    1
of                    1
rapture               1
rhyming               1
ringing               1
swinging              1
tells                 1
that                  1
the                   9

Graphics

Graphics Facilities

• Windows can be opened and closed as desired.
• Text in a variety of type faces and sizes, including proportional-width faces, can be written to awindow in the same manner as text is written to a file.
• Characters from the keyboard can be processed as they are typed, without waiting for a return character.
• Points, lines, polygons, circles, arcs, and smooth curves can be freely intermixed with text.
• Color can be specified by numeric value or descriptive phrase.
• Images can be read from files and written to files.
• Buttons, sliders, and other interface tools are available.

procedure histogram(tbl)

   # sort the table by key

   words := sort(tbl, 3)

   # set the display parameters

   line_height := WAttrib("leading")
   window_height := ((*words / 2) + 1) * line_height
   hist_offset := 100
   scale := 10

   # open a window

   window := WOpen("label=Histogram", "size=300," || window_height)

   # display the information

   y := 0
   while WWrite(window, " ", get(words)) do {
      FillRectangle(window, hist_offset, y + 2,
         scale * get(words), line_height - 2)
      y := y + line_height
      }

  # wait for user to dismiss the window

   until WQuit(window)

   WClose(window)

   return

end

An Example Application

Conclusions

Status and Availability

http://www.cs.arizona.edu/icon/

ftp.cs.arizona.edu

Icon Project
Department of Computer Science
The University of Arizona
P.O. Box 210077
Tucson, AZ85721-0077
U.S.A.

(520) 621-6613 (voice)
(520) 621-4246 (fax)

icon-project@cs.arizona.edu

Acknowledgments

References