POP-11
commands are instructions for the computer to perform some action on a
specified data object. A simple action is to display the object on the screen;
the command to do this is represented by the following symbol, called a
print arrow: =>.
You type it by pressing the = key and then the > key. To display a data
object, you type the object, followed by a print arrow, e.g.,
[The cat sat on the mat] =>
** [The cat sat on the mat]
"cat" =>
** cat
In this book we shall indicate symbols typed in by the user in bold type and
the responses produced by the program in normal type. The two stars merely
indicate that the item has been displayed using the print arrow.
The `pretty print' symbol =>>[+]
will display lists without the outer brackets or the
two stars:
[Welcome to the London Tourist Guide] =>>
Welcome to the London Tourist Guide
You can put
spaces between items. An item is a single data object or symbol (e.g.,
cat
or => ).
Thus, you could write,
[ The cat sat on the mat ] =>
** [The cat sat on the mat]
POP-11
will not interpret the command correctly if you split items (e.g., if
you type = >
rather than
=>
or
c a t
rather than cat).
Simply displaying an object on the screen is not a very exciting demonstration
of the power of the computer, but commands can be applied not only to objects
but also to combinations of objects, called
expressions.
An expression is a series of objects linked by
operators.
One familiar kind is the arithmetic expression, consisting of numbers linked
by arithmetic operators such as + - /
(for `divide')
and
*
(for `multiply').
An expression followed by a print arrow instructs the computer to
evaluate
the expression and display the result, e.g.,
2+2=>
** 4
12 - 3 + 5 =>
** 14
POP-11 interprets a program by carrying out each command in turn, where necessary evaluating the expressions and applying the command to the result. When the last command has been interpreted, the program ends.