CLHS: Issue MAP-INTO Writeup

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## Issue MAP-INTO Writeup

    Status:	Passed, Jun 89 X3J13
    Issue:          MAP-INTO
    References:     none
    Related issues: BIT-ARRAY-FUNCTIONS
    Category:       ADDITION
    Edit history:   23-May-89, version 1 by Moon
                    19-Jun-89, version 2 by Moon (fix arglist)
                    23-Jun-89, version 3 by Moon (include BarMar's suggestions)

    Problem description:

      The function MAP is very useful but can be a source of inefficiency
      because it conses the result.  Sometimes the user has storage
      already allocated in which the result could be stored.

    Proposal (MAP-INTO:ADD-FUNCTION):

      Add the following function:

        MAP-INTO result-sequence function &rest sequences            [Function]

        Destructively modifies the result-sequence to contain the results of
        applying function to each element in the argument sequences in turn.
        Returns result-sequence.

        The arguments result-sequence and each element of sequences can each be
        either a list or a vector (one-dimensional array).  Note that NIL is
        considered to be a sequence, of length zero.  If result-sequence and
        each element of sequences are not all the same length, the iteration
        terminates when the shortest sequence is exhausted.  If result-sequence
        is a vector with a fill-pointer, the fill-pointer is ignored when
        deciding how many iterations to perform, and afterwards the
        fill-pointer is set to the number of times function was applied.

        If result-sequence is longer than the shortest element of sequences,
        extra elements at the end of result-sequence are left unchanged.

        MAP-INTO differs from MAP in that it modifies an existing sequence
        rather than creating a new one.  In addition, MAP-INTO can be called
        with only two arguments, while MAP requires at least three arguments.
        If result-sequence is NIL, MAP-INTO immediately returns NIL, since
        NIL is a sequence of length zero.

        If BIT-ARRAY-FUNCTIONS:NO-NEW-FUNCTIONS passes, then MAP-INTO will
        allow result-sequence and each element of sequences to be mappables
        all of the same rank.

        The function must take at least as many arguments as there are
        sequences provided.

        If function has side effects, it can count on being called first on all
        of the elements with index 0, then on all of those numbered 1, and so
        on.

    Examples:

      (map-into x #'+ x y)
      (map-into q #'cons keys vals)
      (map-into syms #'gensym)

    Rationale:

      MAP-INTO is a simple way to express reuse of storage that is
      stylistically consistent with the rest of Common Lisp.

    Current practice:

      Symbolics Genera 7.2 implements the proposal.

    Cost to Implementors:

      Small.

    Cost to Users:

      None.

    Cost of non-adoption:

      Small.

    Performance impact:

      None.

    Benefits:

      More expressive language.

    Esthetics:

      User programs won't have to write the above examples as

       (loop for xx on x and yy in y do
          (setf (car xx) (+ (car xx) yy)))

      or something else about equally horrible.

    Discussion:

      None.

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