and
and Macro
Syntax:
and {form}* → {result}*
Arguments and Values:
form—a form.
results—the values resulting from the evaluation of the last form, or the symbols nil or t.
Description:
The macro and evaluates each form one at a time from left to right. As soon as any form evaluates to nil, and returns nil without evaluating the remaining forms. If all forms but the last evaluate to true values, and returns the results produced by evaluating the last form.
If no forms are supplied, (and) returns t.
and passes back multiple values from the last subform but not from subforms other than the last.
Examples:
(if (and (>= n 0)
(< n (length a-simple-vector))
(eq (elt a-simple-vector n) ’foo))
(princ "Foo!"))
The above expression prints Foo! if element n of a-simple-vector is the symbol foo, provided also that n is indeed a valid index for a-simple-vector. Because and guarantees left-to-right testing of its parts, elt is not called if n is out of range.
(defparameter temp1 1) → temp1
(defparameter temp2 1) → temp2
(defparameter temp3 1) → temp3
(and (incf temp1) (incf temp2) (incf temp3)) → 2
(and (eql 2 temp1) (eql 2 temp2) (eql 2 temp3)) → true
(decf temp3) → 1
(and (decf temp1) (decf temp2) (eq temp3 ’nil) (decf temp3)) → NIL
(and (eql temp1 temp2) (eql temp2 temp3)) → true
(and) → T
See Also:
cond, every, if, or, when
Notes:
(and *form*) *≡* (let () *form*)
(and *form1 form2* ...) *≡* (when *form1* (and *form2* ...))
We fixed the examples so that it uses defparameter to initialize the variables. and we added a paragraph explaining why and must be defined as a macro, and the consequences of it being defined as a macro.
Expanded Reference: and
and is defined as a macro because it stops evaluating as soon as any form evaluates to nil. If it were defined as a function, every form would be evaluated before the function was applied. As a result of and not being a function, it can not be used with apply. In order to check that every value of a list is true, the function every could be used.