Status: Passed, Jun 89 X3J13
Related issues: BIT-ARRAY-FUNCTIONS
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)
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.
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.
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
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
(map-into x #'+ x y)
(map-into q #'cons keys vals)
(map-into syms #'gensym)
MAP-INTO is a simple way to express reuse of storage that is
stylistically consistent with the rest of Common Lisp.
Symbolics Genera 7.2 implements the proposal.
Cost to Implementors:
Cost to Users:
Cost of non-adoption:
More expressive language.
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.