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.