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:= 2;
> print(`a:`, a, ` eval a:`, eval(a), ` a:`, a);
> end:
> g(2);
2
b + 2
2 2
a:, b + 2, eval a:, 6, a:, b + 2
> # So full evaluation doesn't happen to local variable "a", even
> # if evaluation is called. You'd have to assign the result of
> # "eval(a)" to "a" to update its value.
> a;
0
> # Just to check, the global variable "a" is not affected by
> # the local variable "a" in function "g".
11.6.2.2 Options
Options are used to control run-time behaviour of a procedure. The options defined for Maple V are
builtin, remember, system, operator, and trace. remember is the one of greatest use to most users.
When you use option remember, the values that correspond to particular parameter lists are stored for
future use. This way they can be retrieved instead of recalculated, a feature particularly useful for
recursive function implementations.
You can force remember for particular argument values, whether or not option remember has been
specified. You simply perform an assignment to the function with the appropriate parameter value. For
example:
> h(2) := 3;
h(2) := 3
> h(2);
3
This is why it is necessary to use proc to define functions. It is tempting to try syntactic definitions like
m(x) := x^2, but what you are really doing is stating something like "the value of function m at the
string `x` has the value `x`^2", assuming "x" was a mathematical variable when you entered in the
assignment statement. When you try to plot this pseudo-function, you will find that some forms of
plotting (like parameterized 3-D plotting) do not work.
11.6.2.3 Explicit RETURN
The syntax of the RETURN statement is simply:
RETURN(value);
This causes the procedure to terminate, returning the value specified.
11.7 Error Handling
There are three facilities for error handling in Maple: ERROR, lasterror, and traperror.
11.7.1 ERROR
The ERROR statement is used for error return. Its syntax is simply:
ERROR(error-message-string);
The "ERROR" must be in capital letters. When you use ERROR in procedures to indicate that
something has gone wrong, you will see a message printed of the form:
Error, (in procname) error-message-string
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