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               Extensions to Version 5 of the Icon
                      Programming Language




1.  Introduction

   The standard features of Version 5 of Icon are described in
Reference 1. Since Icon is the byproduct of a research effort
that is concerned with the development of novel programming
language facilities for processing nonnumeric data, it is inevit-
able that some extensions to the standard language will develop.

   Some of these extensions are incorporated as features of new
releases.  Others are available as options that can be selected
when the Icon system is installed. This report describes the
extensions that are included in Version 5.9 of Icon.

   All the extensions are upward-compatible with standard Version
5 Icon.  Their inclusion should not interfere with any program
that works properly under the standard version.


2.  New Version 5.9 Features

2.1  The Link Directive

   Version 5.9 contains a link directive that simplifies the
inclusion of separately translated libraries of Icon procedures.
If icont is run with the -c option, source files are translated
into intermediate ucode files (with names ending in .u1 and .u2).
For example,

        icont -c libe.icn

produces the ucode files libe.u1 and libe.u2. The ucode files can
be incorporated in another program with the new link directive,
which has the form

        link libe

The argument of link is, in general, a list of identifiers or
string literals that specify the names of files to be linked
(without the .u1 or .u2). Thus, when running under UNIX*,

        link libe, "/usr/icon/ilib/collate"

specifies the linking of libe in the current directory and col-
late in /usr/icon/ilib. Syntax appropriate to VMS should be used
when running under that system.

*UNIX is a trademark of AT&T Bell Laboratories.




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   The environment variable IPATH controls the location of files
specified in link directives. IPATH should be have a value of the
form p1:p2: ... pn where each pi names a directory.  Each direc-
tory is searched in turn to locate files named in link direc-
tives. The default value of IPATH is ".", that is, the current
directory.

2.2  Installation Options

   When an Icon system is installed, various configuration
options are specified [2]. The value of the keyword &options is a
string that contains the command line arguments that were used to
configure Icon.


3.  Optional Extensions

   There are two extension options: sets (-sets in &options), and
a collection of experimental features (-xpx in &options).

3.1  Sets

   Sets are unordered collections of values and have the proper-
ties normally associated with sets in the mathematical sense.
The function

        set(a)

creates a set that contains the distinct elements of the list a.
For example,

        set(["abc",3])

creates a set with two members, abc and 3.  Note that

        set([])

creates an empty set.  Sets, like other data aggregates in Icon,
need not be homogeneous -- a set may contain members of different
types.

   Sets, like other Icon data aggregates, are represented by
pointers to the actual data. Sets can be members of sets, as in

        s1 := set([1,2,3])
        s2 := set([s1,[]])

in which s2 contains two members, one of which is a set of three
members and the other of which is an empty list.

   Any specific value can occur only once in a set. For example,

        set([1,2,3,3,1])




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creates a set with the three members 1, 2, and 3.  Set membership
is determined the same way the equivalence of values is deter-
mined in the operation

        x === y

For example,

        set([[],[]])

creates a set that contains two distinct empty lists.

   The functions and operations of Icon that apply to other data
aggregates apply to sets as well. For example, if s is a set,

        *s

is the size of s (the number of members in it). Similarly,

        type(s)

produces the string set and

        s := set(["abc",3])
        write(image(s))

writes set(2). Note that the string images of sets are in the
same style as for other aggregates, with the size enclosed in
parentheses.

   The operation

        !s

generates the members of s, but in no predictable order. Simi-
larly,

        ?s

produces a randomly selected member of s.  These operations pro-
duce values, not variables -- it is not possible to assign a
value to !s or ?s.

   The function

        copy(s)

produces a new set, distinct from s, but which contains the same
members as s. The copy is made in the same fashion as the copy of
a list -- the members themselves are not copied.

   The function





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        sort(s)

produces a list containing the members of s in sorted order.
Sets themselves occur after tables but before records in the
sorting order.

   The customary set operations are provided. The function

        member(s,x)

succeeds and returns the value of x if x is a member of s, but
fails otherwise. Note that

        member(s1,member(s2,x))

succeeds if x is a member of both s1 and s2.

   The function

        insert(s,x)

inserts x into the set s and returns the value of s (it is simi-
lar to put(a,x) in form). Note that

        insert(s,s)

adds s as an member of itself.

   The function

        delete(s,x)

deletes the member x from the set s and returns the value of s.

   The functions insert(s,x) and delete(s,x) always succeed,
whether or not x is in s. This allows their use in loops in which
failure may occur for other reasons. For example,

        s := set([])
        while insert(s,read())

builds a set that consists of the (distinct) lines from the stan-
dard input file.

   The operations

        s1 ++ s2
        s1 ** s2
        s1 -- s2

create the union, intersection, and difference of s1 and s2,
respectively. In each case, the result is a new set.

   The use of these operations on csets is unchanged. There is no



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automatic type conversion between csets and sets; the result of
the operation depends on the types of the arguments. For example,

        'aeiou' ++ 'abcde'

produces the cset abcdeiou, while

        set([1,2,3]) ++ set([2,3,4])

produces a set that contains 1, 2, 3, and 4. On the other hand,

        set([1,2,3]) ++ 4

results in Run-time Error 119 (set expected).

Examples

Word Counting:

   The following program lists, in alphabetical order, all the
different words that occur in the standard input file:

        procedure main()
           letter := &lcase ++ &ucase
           words := set([])
           while text := read() do
              text ? while tab(upto(letter)) do
                 insert(words,tab(many(letter)))
           every write(!sort(words))
        end


The Sieve of Eratosthenes:

   The follow program produces prime numbers, using the classical
"Sieve of Eratosthenes":

        procedure main(a)
           local limit, s, i
           limit := a[1] | 5000            # limit to 5000 if not specified
           s := set([])
           every insert(s,1 to limit)
           every member(s,i := 2 to limit) do
              every delete(s,i + i to limit by i)
           primes := sort(s)
           write("There are ",*primes," primes in the first ",limit," integers.")
           write("The primes are:")
           every write(right(!primes,*limit + 1))
        end








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4.  Expermental Features

4.1  PDCO Invocation Syntax

   The experimental features include the procedure invocation
syntax that is used for programmer-defined control operations
[3].  In this syntax, when braces are used in place of
parentheses to enclose an argument list, the arguments are passed
as a list of co-expressions. That is,

        p{expr1, expr2, ..., exprn}

is equivalent to

        p([create expr1, create expr2, ..., create exprn])

Note that

        p{}

is equivalent to

        p([])


4.2  Invocation Via String Name

   The experimental features allow a string-valued expression
that corresponds to the name of a procedure or operation to be
used in place of the procedure or operation in an invocation
expression. For example,

        "image"(x)

produces the same call as

        image(x)

and

        "-"(i,j)

is equivalent to

        i - j


   In the case of operations, the number of arguments determines
the operation. Thus

        "-"(i)

is equivalent to




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        -i

Since to-by is an operation, despite its reserved-word syntax, it
is included in this facility with the string name ... .  Thus

        "..."(1,10,2)

is equivalent to

        1 to 10 by 2

Similarly, range specifications are represented by ":", so that

        ":"(s,i,j)

is equivalent to

        s[i:j]


   Defaults are not provided for omitted or null-valued arguments
in this facility. Consequently,

        "..."(1,10)

results in a run-time error when it is evaluated.

   The subscripting operation also is available with the string
name []. Thus

        "[]"(&lcase,3)

produces c.

   String names are available for the operations in Icon, but not
for control structures. Thus

        "|"(expr1,expr2)

is erroneous.  Note that string scanning is a control structure.
In addition, conjunction is not available via string invocation,
since no operation is actually performed.

   Field references, of the form

        expr . fieldname

are not operations in the ordinary sense and are not available
via string invocation.

   String names for procedures are available through global iden-
tifiers.  Note that the names of functions, such as image, are
global identifiers. Similarly, any procedure-valued global iden-
tifier may be used as the string name of a procedure. Thus in



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        global q

        procedure main()
           q := p
           "q"("hi")
        end

        procedure p(s)
           write(s)
        end

the procedure p is invoked via the global identifier q.

4.3  Conversion to Procedure

   The experimental features include the function proc(x,i),
which converts x to a procedure, if possible.  If x is
procedure-valued, its value is returned unchanged. If the value
of x is a string that corresponds to the name of a procedure as
described in the preceding section, the corresponding procedure
value is returned.  The value of i is used to distinguish between
unary and binary operators.  For example, proc("^",2) produces
the exponentiation operator, while proc("^",1) produces the co-
expression refresh operator.  If x cannot be converted to a pro-
cedure, proc(x,i) fails.

4.4  Integer Sequences

   To facilitate the generation of integer sequences that have no
limit, the experimental features include the function seq(i,j).
This function has the result sequence {i, i+j, i+2j, ... }. Omit-
ted or null values for i and j default to 1. Thus the result
sequence for seq() is {1, 2, 3, ... }.

Acknowledgements

   Rob McConeghy and Bill Mitchell made major contributions to
the design and implementation of Version 5.9.


   The design of sets for Icon was done as part of a class pro-
ject.  The following persons participated in the design:  John
Bolding, Owen Fonorow, Roger Hayes, Tom Hicks, Robert Kohout,
Mark Langley, Rob McConeghy, Susan Moore, Maylee Noah, Janalee
O'Bagy, Gregg Townsend, and Alan Wendt.

References

1.  Griswold, Ralph E. and Madge T. Griswold. The Icon Program-
ming Language, Prentice-Hall, Inc., Englewood Cliffs, New Jersey.
1983.


2.  Griswold, Ralph E. The Translation and Execution of Icon



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Programs under MS-DOS, Technical report, Department of Computer
Science, The University of Arizona.  September 1985.

3.  Griswold, Ralph E. and Michael Novak. "Programmer-Defined
Control Operations", The Computer Journal, Vol. 26, No. 2 (May
1983).  pp. 175-183.




Ralph E. Griswold
Department of Computer Science
The University of Arizona

September 30, 1985










































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