ref: 7d02e382d314d5bdde7978ccb7a64ea9201d03db
dir: /doc/source/locals.ht/
<?
ficlPageHeader("local variables in Ficl")
ficlAddToNavBarAs("Locals")
def entry(definition, description):
print "<tr><td bgcolor=#e0e0e0>\n<b>" + definition + "</b>\n</td><td bgcolor=#f0f0f0>\n" + description + "\n</td></tr>\n"
?>
<? ficlHeader1("An Overview And A History") ?>
Named, locally scoped variables came late to Forth. Purists feel that experienced
Forth programmers can (and should) write supportable code using only anonymous
stack variables and good factoring, and they complain that novices use
global variables too frequently. But local variables cost little in terms of
code size and execution speed, and are very convenient for OO programming
(where stack effects are more complex).
<p>
Ficl provides excellent support
for local variables, and the purists be damned—we use 'em all the time.
<p>
Local variables can only be declared inside a definition,
and are only visible in that definition. Please refer to
<a href="http://ficl.sourceforge.net/dpans/dpans13.htm">
the ANS standard for FORTH
</a> for more general information on local variables.
<? ficlHeader1("John-Hopkins Forth Argument Syntax") ?>
ANS Forth does not specify a complete local variable facility.
Instead, it defines a foundation upon which to build one. Ficl comes with
an adaptation of the Johns-Hopkins local variable syntax, as developed by John
Hayes et al. However, Ficl extends this syntax with support for double-cell and
floating-point numbers.
<p>
Here's the basic syntax of a JH-local variable declaration:
<blockquote><code>
<b>{</b> <i>arguments</i>
<b>|</b> <i>locals</i>
<b>--</b> <i>ignored</i>
<b>}</b>
</code></blockquote>
(For experienced FORTH programmers: the declaration is designed to look like a stack comment,
but it uses curly braces instead of parentheses.) Each section must list zero or more
legal Ficl word names; comments and preprocessing are not allowed here.
Here's what each section denotes:
<ul>
<li>
The <i>arguments</i> section lists local variables which are initialized from the stack when the word executes.
Each argument is set to the top value of the stack, starting at the rightmost argument name and moving left.
You can have zero or more arguments.
<p>
<li>
The <i>locals</i> section lists local variables which are set to zero when the word executes.
You can have zero or more locals.
<p>
<li>
Any characters between <code>--</code> and <code>}</code> are treated as a comment, and ignored.
</ul>
(The <code>|</code> and <code>--</code> sections are optional,
but they must appear in the order shown if they appear at all.)
<p>
<? ficlHeader2("Argument Types") ?>
Every time you specify a local variable (in either the <i>arguments</i> or the <i>locals</i> section),
you can also specify the <i>type</i> of the local variable. By default, a local variable
is a single-cell integer; you can specify that the local be a double-cell integer, and/or a
floating-point number.
<p>
To specify the type of a local, specify one or more of the following single-character specifiers,
followed by a colon (<code>:</code>).
<table>
<? entry("1", "single-cell") ?>
<? entry("2", "double-cell") ?>
<? entry("d", "double-cell") ?>
<? entry("f", "floating-point (use floating stack)") ?>
<? entry("i", "integer (use data stack)") ?>
<? entry("s", "single-cell") ?>
</table>
For instance, the argument <code>f2:foo</code> would specify a double-width floating-point
number.
<p>
The type specifiers are read right-to left, and when two specifiers conflict, the rightmost
one takes priority. So <code>2is1f2:foo</code> would still specifiy a double-width floating-point
number.
<p>
Note that this syntax <i>only works</i> for Ficl's JH-locals. Locals
defined in some other way (say, with the FORTH standard word <code>LOCALS|</code>)
will ignore this syntax, and the entire string will be used as the name of
the local (type and all).
<? ficlHeader2("A Simple Example") ?>
<pre>
: DEMONSTRATE-JH-LOCALS { c b a f:float -- a+b f:float*2 }
a b +
2.0e float f*
;
</pre>
<?
ficlPageFooter()
?>