More on the XLISP-STAT Listener

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More on the XLISP-STAT Listener

Because of the large number of parentheses involved, Lisp expressions can be hard to read and type correctly. To make it easier to type readable, correct expressions the listener window on the Macintosh has the following features:

Typing a closing parenthesis flashes the matching opening parenthesis.
You can move the cursor with the arrow keys, the mouse or the backspace key to any position in the current input expression, not just within the last line.
If the current expression is more than one line long, hitting the tab key in any line but the first indents the line to its appropriate position according to (more or less) standard rules for Lisp code indentation.
If the insertion point is at the end of the current expression hitting the enter key is equivalent to hitting a return. If the insertion point is not at the end of the expression, hitting enter moves the insertion point to the end of the expression.

These four features should make typing expressions correctly much easier. In particular, in translating mathematical formulas to Lisp it sometimes seems that you have to do things backwards. Using these features you can build up your expression from the inside out

XLISP 2.0 provides a simple command history mechanism. The symbols -, *, **, ***, +, ++, and +++ are used for this purpose. The top level reader binds these symbols as follows:

The variables *, ** and *** are probably most useful. For example, if you construct a plot but forget to assign the resulting plot object to a variable you can recover it using one of the history variables:

> (histogram (normal-rand 50))
#<Object: 3701682, prototype = HISTOGRAM-PROTO>
> (def w *)
W
> w
#<Object: 3701682, prototype = HISTOGRAM-PROTO>
>

The symbol W now has the histogram object as its value and can be used to modify the plot, as described in Section 6.5 below.

Like most interactive systems, XLISP needs a system for dynamically managing memory. The system used by XLISP is to grab memory out of a fixed bin until the bin is exhausted. At that point the system pauses to reclaim memory that is no longer being used. This process, called garbage collection, will occasionally cause the system to freeze up for about a second. When the system garbage collects the Macintosh cursor changes to a trash bag.

Occasionally a calculation will take too long, or it will appear to have gotten stuck in some kind of loop. If you want to interrupt the calculation hold down the COMMAND key and the PERIOD. This should return you to the listener. You must continue to hold down the key until the calculation stops.

Next: Loading Files Up: Some Useful Shortcuts Previous: Listing and Undefining

Luke Tierney
Tue Jan 21 15:04:48 CST 1997