Quotes, Names, and Values

There are three types of quotes used in Maple: double-quotes (");
backquotes (`); and apostrophes (').  Multiple double-quotes are used to
refer to the results of previous statements, for example:

> z1 := z2 + z3;
z1 := z2 + z3;
> a1 := a2 * a3;
a1 := a2 * a3;
> " + "";
a2 a3 + z2 + z3

Backquotes (left-quotes) are used to construct strings, as in:

> a := `This is a string`;
a := This is a string
> a;
This is a string

In the previous section, a mathematical variable was also described as
an unassigned variable.  Unassigned variables have their own name as
their value. This is the default condition for a variable until you
assign it a value.  To turn an assigned variable (i.e., a programming
variable) back into an unassigned variable (i.e., a mathematical
variable), you must assign it its own name.  As an example:

> # r hasn't been used, so its an unassigned variable:
> r;
r
> # Now make r an assigned variable:
> r := 2;
r := 2
> r;
2
> # Now try to give r its own name:
> r := r;
r := 2
> # That didn't work, because the r on the right side of the assignment
> # statement was evaluated prior to the assignment.  Assign the name
> # of the variable by delaying evaluation:
> r := 'r';
r := r
> r;
r
> # r is now a mathematical variable once more.

This shows the function of the apostrophe (right-quote).  It delays
evaluation of an expression.

Maple generally performs what is called "full evaluation".  You have
seen an example of this already.  Here it is again:

> z := 2+y;
z := 2 + y
> y := 5;
y := 5
> z;
7

Once "y" had been assigned a value, you didn't have to do anything to
tell Maple that "z" could be completely evaluated to a number instead of
an algebraic expression.  Every time Maple evaluates a statement, it
checks to see if it knows any other expressions that could be
re-evaluated in light of the new information.  An exception to automatic
full evaluation is in procedures, which is discussed later.