BSD 4_2 development

Work on file usr/src/new/new/icon/doc/extensions

Synthesized-from: CSRG/cd1/4.2

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+.de Ds
+.DS
+.ft B
+.ss 9
+..
+.de De
+.DE
+.ss 4
+.ft R
+..
+.sv 1.25i
+.LP
+.ce 100
+\fBExperimental Extensions to Version 5 of Icon\fR
+.sp 2
+Ralph E. Griswold and William H. Mitchell
+.sp 1
+Department of Computer Science, The University of Arizona
+.sp 1
+March 3, 1983
+.sp 2
+.PP
+A number of experimental extensions have been made to Version 5 of
+Icon.
+This version is called Version 5x for reference.
+.PP
+The extensions in Version 5x, which are described below, are
+upward-compatible ones and should not interfere with programs
+that run properly on Version 5.
+.NH
+PDCO Invocation Syntax
+.PP
+Version 5x contains the procedure invocation syntax that is
+used for programmer-defined control operations (see TR 82-8 and
+TR 82-16).
+.PP
+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,
+.Ds
+p{\*1, \*2, \*(el, \*n}
+.De
+is equivalent to
+.Ds
+p([create \*1, create \*2, \*(el, create \*n])
+.De
+There is always at least one co-expression in the list:
+.Ds
+p{\^}
+.De
+is equivalent to
+.Ds
+p([create &null])
+.De
+.NH
+Invocation Via String Name
+.PP
+In the Version 5x interpreter (but not compiler), a string-valued expression that corresponds to the
+name of a procedure or operation may be used in place of the
+procedure or operation in an invocation expression. For example,
+.Ds
+"image"(x)
+.De
+produces the same call as
+.Ds
+image(x)
+.De
+and
+.Ds
+"-"(i,\*bj)
+.De
+is equivalent to
+.Ds
+i - j
+.De
+.PP
+In the case of operations, the number of arguments determines
+the operation. Thus
+.Ds
+"-"(i)
+.De
+is equivalent to
+.Ds
+-i
+.De
+Since \fBto-by\fR is an operation, despite its reserved-word syntax,
+it is included in this facility with the string name \fB...\fR .
+Thus
+.Ds
+"..."(1,\*b10,\*b2)
+.De
+is equivalent to
+.Ds
+1 to 10 by 2
+.De
+Similarly, range specifications are represented by \fB":"\fR, so that
+.Ds
+":"(s,i,j)
+.De
+is equivalent to
+.Ds
+s\^[i:j]
+.De
+.PP
+Defaults are not provided for omitted or null-valued arguments in this
+facility. Consequently,
+.Ds
+"..."(1,\*b10)
+.De
+results in a run-time error when it is evaluated.
+.PP
+The subscripting operation also is available with the string name
+\fB[\^]\fR. Thus
+.Ds
+"[\^]"(&lcase,\*b3)
+.De
+produces \fBc\fR.
+.PP
+String names are available for all the operations in Icon, but not for
+control structures. Thus
+.Ds
+"|"(\*1,\*b\*2)
+.De
+is erroneous.
+.PP
+String names for procedures are available through global identifiers.
+Note that the names of functions, such as \fBimage\fR are global
+identifiers. Similarly, any procedure-valued global identifier may be
+used as the string name of a procedure. Thus in
+.Ds
+global q
+
+procedure main()
+   q := p
+   "q"("hi")
+end
+
+procedure p(s)
+   write(s)
+end
+.De
+the procedure \fBp\fR is invoked via the global identifier \fBq\fR.
+.NH
+Integer Sequences
+.PP
+To facilitate the generation of integer sequences that have no limit,
+the function \fBseq(i,\*bj)\fR has been added. This function has the
+result sequence {\^i, i+j, i+2j, \*(el }. Omitted or null values for \fBi\fR
+and \fBj\fR
+default to 1. Thus the result sequence for \fBseq()\fR is
+{\^1, 2, 3, \*(el }.