

                                                                     iterate

    MACRO
ITERATE ({({VAR | ({VAR}*)} VALUE)}*) {DECLARATION}* {FORM}*        [Macro]

    Package
    series

    DESCRIPTION

The form ITERATE is the same as MAPPING, except that after mapping the
FORMS over the VALUES, the results are discarded and NIL is returned.

(LET ((ITEM (SCAN '((1) (-2) (3))))) 
  (ITERATE ((X (#MCAR ITEM))) 
    (IF (PLUSP X) (PRIN1 X)))) 
 => NIL (after printing ``13'')


To a first approximation, ITERATE and MAPPING differ in the same way as
MAPC and MAPCAR.  In particular, like MAPC, ITERATE is intended to be used
in situations where the FORMS are being evaluated for side effect rather
than their results.  However, due to the lazy evaluation semantics of
series, the difference between ITERATE and MAPPING is more than just a
question of efficiency.

If MAPCAR is used in a situation where the output is not used, time is
wasted unnecessarily creating the output list.  However, if MAPPING is used
in a situation where the output is not used, no computation is performed,
because series elements are not computed until they are used.  Thus ITERATE
can be thought of as a declaration that the indicated computation is to be
performed even though the output is not used for anything.

     SEE ALSO
     about-series
     about-generators
     mapping map-fn

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