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In the previous section, the definition of for was fixed as
follows to make the expansion evaluate the macro arguments the proper
number of times:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
`(let ((,var ,init)
(max ,final))
(while (<= ,var max)
,@body
(inc ,var))))
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The new definition of for has a new problem: it introduces a
local variable named max which the user does not expect. This
causes trouble in examples such as the following:
(let ((max 0))
(for x from 0 to 10 do
(let ((this (frob x)))
(if (< max this)
(setq max this)))))
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The references to max inside the body of the for, which
are supposed to refer to the user's binding of max, really access
the binding made by for.
The way to correct this is to use an uninterned symbol instead of
max (see section 8.3 Creating and Interning Symbols). The uninterned symbol can be
bound and referred to just like any other symbol, but since it is
created by for, we know that it cannot already appear in the
user's program. Since it is not interned, there is no way the user can
put it into the program later. It will never appear anywhere except
where put by for. Here is a definition of for that works
this way:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(let ((tempvar (make-symbol "max")))
`(let ((,var ,init)
(,tempvar ,final))
(while (<= ,var ,tempvar)
,@body
(inc ,var)))))
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This creates an uninterned symbol named max and puts it in the
expansion instead of the usual interned symbol max that appears
in expressions ordinarily.
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