[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

SRFI-1 round 2 discussion



After all of the feedback I got on SRFI-1, I was able to sit down this
week, go over it all carefully, and write up a document containing all
the issues and my proposals wrt each one. I am maintaining the issues document
at
    ftp://ftp.ai.mit.edu/people/shivers/srfi-1
and it is also appended to this message.

As you read through these topics, if you don't feel like declaiming or
writing long, thoughtful essays on the design decisions, but nonetheless
do have an opinion and would like to vote, then do just that: send me a
simple vote. I will tabulate them. The easiest way is to read through
the document, then grab the topic list at the front and annotate each
line with your vote or opinion.

Thank you all for your reviews and comments; sorry for the delay in the loop;
I look forward to the next round.
    -Olin

-------------------------------------------------------------------------------
This is the list of the ongoing issues. For each issue, I list some of the
people who have raised it, and quote some of the email to sum up various
points of view.

In discussion, please refer to the relevant topic by its tag or header. 
That will help us stay organised as we range over a lot of different
issues.

I am pretty sure that the draft reference implementation no longer precisely
conforms to the spec as modified by these topics. Once discussion has
converged, I will hack the ref imp into conformance.

Here is a list of 26 current issues/discussion topics I have identified from
the email. Some are minor or simple; some are rather difficult.  You may wish
to read only the topics about which you care.  To aid navigation, this
document format can be parsed using emacs' outline mode.
    Add LIST-DIFFERENCE ?
    iota defn
    Naming: iota
    circular lists
    improper lists
    TAKE & DROP
    Add SUBLIST ?
    Removing PROPER-LIST?
    map function termination condition
    FIND, FIND-TAIL n-ary
    alist functions in separate lib?
    FIND-TAIL applies pred to list cells or list elts?
    Are the right-duplicate deletion procedures worth inclusion?
    lists-as-sets funs put in separate module?
    Naming: REMOVE / DELETE conflicts
    More careful specification of error cases
    Argument order for FOLDL and FOLDR
    Naming: right & left variants
    Add UNZIP1 ?
    destructive/linear-update
    Naming: ACONS or ALIST-CONS?
    Naming: PAIR-frob prefix vs frob-TAIL suffix 
    MAKE-LIST's default fill argument
    Naming: CONS* or LIST*
    Naming: APPEND-REVERSE{!} or REVERSE-APPEND{!}
    Argument order of = equivalence predicates

This document, along with current drafts of the reference implementation
and the draft SRFI (in ASCII format) can be found at
    ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/
I'll HTML'ize them for the final SRFI format when discussion is done.

A related document contains minor issues -- typos, things I went ahead
and changed without feeling they required discussion:
    ftp://ftp.ai.mit.edu/people/shivers/srfi/srfi-1/small-stuff.txt

-Olin

-------------------------------------------------------------------------------
* Add LIST-DIFFERENCE ?

Several people have asked for an LDIFF or LIST-DIFFERENCE function.
I asked for examples of its use. After consideration of the function,
and the examples sent me, I am against adding this routine.

I have examined all the examples people have sent me of uses of
LDIFF or LIST-DIFFERENCE. Without fail, they can be rewritten
using other tools for more general or efficient implementations.
In general, I have come to associate LIST-DIFFERENCE with sloppy
coding.

However, even if one is attached to some particular use of LIST-DIFFERENCE,
it can always be written trivially in terms of UNFOLD:
    (LIST-DIFFERENCE A B)
is equivalent to
    (UNFOLD (LAMBDA (L) (EQ? L B)) CAR CDR A)
However, the UNFOLD idiom is not committed to the EQ? equality predicate.
(I usually take a commitment to a particular equality predicate as a
sign of poor design.)

The following PERMUTE function is a frequent example given, credited to Duncan
Smith (note that the versions I got all produced the buggy base case 
(permute '()) => (), instead of (()).

(define (permute ls)
  (cond ((null ls) '(()))			; Was buggy.
	((null? (cdr ls)) (list ls))
	(else (mapcon (lambda (x)
			(mapcar (lambda (y) (cons (car x) y))
				(permute (nconc (ldiff ls x)
						(cdr x)))))
		      ls))))

With four more lines of code, we can eliminate the redundant scanning
and rescanning performed by the LDIFF, for a much more efficient 
implementation:

(define (permute ls)
  (if (pair? ls)
      (let lp ((rev-head '()) (tail ls) (ans '()))
	(if (pair? tail)
	    (let ((x    (car tail))
		  (tail (cdr tail)))
	      (lp (cons x rev-head) tail
		  (foldl (lambda (perm ans) (cons (cons x perm) ans))
			 ans
			 (permute (reverse-append rev-head tail)))))
	    ans))
      '(())))


LIST-DIFFERENCE is sometimes used to parse lists with an infix 
separator element, as in this example, where we have a list of the form
    (x1 ... xn => y1 ... ym)
Here's code that splits a list of this form into the pre-=> elements
and the post-=> elements:

(define (parse-signature spec)
  (let ((mid (memq '- spec)))
    (values (list-difference spec mid)
	    (cdr mid))))

Now, we can always just replace the 
    (LIST-DIFFERENCE SPEC MID) 
with the equivalent
    (UNFOLD (LAMBDA (L) (EQ? L MID)) CAR CDR SPEC)
However, with two more lines of extra code, we can rewrite PARSE-SIGNATURE
without LIST-DIFFERENCE or MEMQ, giving an implementation that runs twice
as fast:

(define (parse-signature spec)
  (let recur ((elts spec))
    (let ((elt (car elts)))
      (if (eq? elt '-) (values '() (cdr elts))
	  (receive (front tail) (recur (cdr elts))
	    (values (cons elt front) tail))))))
    
Or, better yet, abstract out the pattern of "splitting at some element"
into this routine:

(define (split-at lis split?)
  (let recur ((elts spec))
    (let ((elt (car elts)))
      (if (split? elt) (values '() (cdr elts))
	  (receive (front tail) (recur (cdr elts))
	    (values (cons elt front) tail))))))

...and then define our PARSE-SIGNATURE function in terms of it:

(define (parse-signature spec)
  (receive (front tail) (split-at spec (lambda (x) (eq? x '=>)))
    (values front (cdr tail))))


Similarly, we can use LIST-DIFFERENCE to save one line of code
defining an all-but-the-last-element BUTLAST function. Writing
the function without LIST-DIFFERENCE costs one extra line of code
and runs twice as fast.

(define (butlast L)
  (list-difference L (last-pair L)))

(define (butlast lis)
  (let recur ((x (car lis)) (l (cdr lis)))
    (if (pair? l) (cons x (recur (car l) (cdr l))) '())))

LIST-DIFFERENCE is a loser.

-------------------------------------------------------------------------------
* iota defn

From: Donovan Kolbly <donovan@xxxxxxxxxxx>
From: Doug Evans <devans@xxxxxxxxxx>
To: srfi-1@xxxxxxxxxxxxxxxxx
Subject: .iota/iota.

Some have suggested an alternate definition for an iota function:
    (iota count [start step]) ; start=0; step=1

Evans' proposed IOTA is a fine function. Let's be clear about the difference.
His iota function is *count* based -- you say how many samples you want.  The
functions I have proposed are *bounds* based -- you say that the samples run
over a particular half-open interval, and the function figures out how many
samples are needed.

While the proposed IOTA is simpler than the :IOTA and IOTA: functions I've
proposed, in many cases, this just puts the burden of calculation back on the
programmer -- where there is potential for error. Calculating the proper
number of samples is a simple bit of arithmetic, but it's easy to get wrong.
Fencepost errors, getting the floor/ceiling distinction wrong -- there are two
or three little things that can blow you out of the water, and they come up
each time you use the function. So the nice thing about the :IOTA and IOTA:
functions is that you simply say what you want, and the functions give you the
samples.

I don't intend to fight this one to the death, in part because IOTA is largely
for interactively fooling around. I see three possibilities, and would like to
know how people think:

- (IOTA count [start step]) only
- My :IOTA and IOTA: only
- All three procedures

I am kinda partial to choice 3. Some may feel differently purely on bloat
grounds. Also, going with all 3 raises a naming problem.  The naming mnemonic
with :IOTA and IOTA: is that a colon on the left or right indicates that the
corresponding interval bound is inclusive -- :IOTA includes the left bound,
and IOTA: includes the right bound. Unfortunately, IOTA fits into this scheme
-- as the fully-open interval, just as :IOTA: might mean the fully-closed
interval. So the proposed IOTA function violates the naming scheme that comes
along with :IOTA and IOTA:. If we go with choice 3, we need a set of three
short, convenient names for these functions that fit together.

I have implemented Evans' IOTA and stuck it into the reference lib & spec.
So it will be pretty easy to do any renaming or deletions on which we decide.

More discussion, please?

-------------------------------------------------------------------------------
* Naming: iota
John David Stone <stone@xxxxxxxxxxxxx>
Harvey Stein
Sergei Egorov <esl@xxxxxxxxxxxxxxx>
Donovan Kolbly <donovan@xxxxxxxxxxx>

Several people pointed out that .IOTA and IOTA. are not legal R5RS identifiers
(which is really irritating -- it's a ridiculuous design misfeature in the
current spec). So I have replaced these two bindings with
    :IOTA 
    IOTA:
These *are* legal R5RS identifiers. However, see the previous issue
("iota defn") for related discussion.

-------------------------------------------------------------------------------
* circular lists

From: Doug Currie <e@xxxxxxxxxxx>
    2 General Comment: The srfi should say which procedures work on improper
    lists and also which work on circular lists. A quick glance at the model
    implementation indicates to me that you did not intend the procedures to
    work on circular lists; the stfi should say so. On the other hand, maybe
    some procedures should work on circular lists (especially list-length, and
    maybe list-copy).

Good point. Here is my proposed taxonomy, which corresponds pretty
closely to the natural, straightforward definitions of these functions,
with some room left in for fancier implementations:

Works on circular lists: 
   xcons cons*
   first ... tenth
   Taking and dropping from the left
   zip with at least one finite list
   append append! reverse-append reverse-append! -- last arg may be circular
   acons
   cons pair? null? list?
   car cdr ... cdddar cddddr set-car! set-cdr! list-ref

May diverge / is an error / bad thing:
(Plus'd entries have meaningful definitions for circular lists; we might
 leave these cases open to the implementation. Discussion?)
+  list-copy
   Taking and dropping from the right
   last last-pair
   zip with no finite list
   append append! reverse-append reverse-append! -- initial args must be finite
+  unzip2 unzip3 unzip4 unzip5
+  filter  partition  remove
+  filter! partition! remove! 
+  del  delq  delv  delete 
+  del! delq! delv! delete!
+  alist-copy
+  delq-duplicates  delv-duplicates  delete-duplicates  del-duplicates 
+  delq-duplicates! delv-duplicates! delete-duplicates! del-duplicates!
+  alist-delete  del-ass  del-assq  del-assv  del-assoc
+  alist-delete! del-ass! del-assq! del-assv! del-assoc!
   reverse!
   length reverse 

Some lists may be circular; at least one must be finite:
   foldl foldr pair-foldl pair-foldr reducel reducer
   append-map append-map! map! pair-for-each filter-map map-in-order
   map for-each

May diverge if no hit:
(Some implementations might do a fancy implementation 
and *not* diverge. Sure.)
   find find-tail any every list-index
   mem member memq memv ass assoc assq assv


-------------------------------------------------------------------------------
* improper lists

The issue has been raised (see, for example, Currie's email quoted in
topic "circular lists") of what the list-lib procedures do when applied
to improper lists.

I try for an "accept a wide spectrum" approach to the inputs the list
functions take, and intend to spec as many of these procedures to work on
improper lists as possible. Procedures that work on improper lists must be
able to treat any non-pair as a list-tail signifying the end of the list --
that is, the empty list. E.g. a robust LENGTH function that handles improper
lists must be spec'd so that both of these calls return three:
    (length '(a b c . ())) => 3		; Proper
    (length '(a b c . d))  => 3		; Improper
This means that *any non-pair* must be treated as the empty list. This
fact falls out naturally from the straightforward base cases of these
recursive functions. Hence
    (length '()) => 0	; Proper
    (length 'd)  => 0	; Improper
This is the simplest, most consistent spec that covers improper lists.
The functions in the current reference implementation have this property.

In this spirit, *all* of the procedures in list-lib are defined to work
on improper lists. 

As a related design criteria, I am specifying these procedures to replicate
the proper/improperness of their args in their results where this can
straightforwardly be defined. So, for example:
    (filter even? '(2 7 1 8 2))       => (2 8 2)
    (filter even? '(2 7 1 8 2 . end)) => (2 8 2 . end)

There is a trade-off here. On one hand, widening the set of arguments
acceptable to our utilities reduces our ability to provide error-checks
when an improper list means an error. On the other, it widens the
applicability of these procedures. I tilt towards the latter. Scheme
supports improper lists; so I will support them. Were this ML, the issue
would not arise -- tuple-trees and lists are statically distinguished by
the type system.

Replicates the argument's terminating value:
    list-copy [e.g., (list-copy '(a b . c)) => (a b . c)]
    filter  partition  remove
    filter! partition! remove! 
    del  delq  delv  delete 
    del! delq! delv! delete!
    alist-copy
    delq-duplicates  delv-duplicates  delete-duplicates  del-duplicates 
    delq-duplicates! delv-duplicates! delete-duplicates! del-duplicates!
    alist-delete  del-ass  del-assq  del-assv  del-assoc
    alist-delete! del-ass! del-assq! del-assv! del-assoc!
    take take! (from right) drop drop! (from left)
    member memq memv assoc assq assv
    map filter-map map-in-order map!
        -- These n-ary funcs use the terminator from left-most shortest list
        arg. Simple to implement, and produces the desired result in the
        single-list map case. Example:
	    (map + '(1 2 3 4 . a) '(1 0 1 . b) '(0 1 0 . c)) =>
                (2 3 4 . b)
    unzip2 unzip3 unzip4 unzip5

Not specified (may either replicate or nil-terminate):
    reverse! 

Produces nil-terminated result:
    take take! (from left, always) 
    drop drop! (from right, always)
    reverse

Handles improper-list args, but issue of nil-termination does not
apply to result or is trivial:
    xcons tree-copy make-list list-tabulate list* 
    circular-list :iota iota:
    first second third fourth fifth sixth seventh eighth ninth tenth
    append append! reverse-append reverse-append!
    unfold unfold/tail
    pair-for-each append-map append-map! 
    foldl foldr pair-foldl pair-foldr reducel reducer
    find find-tail any every list-index
    mem ass acons 
    last last-pair
    zip
    cons pair? null? list? list length 
    car cdr ... cdddar cddddr set-car! set-cdr! list-ref for-each

-------------------------------------------------------------------------------
* TAKE & DROP

From: Doug Currie <e@xxxxxxxxxxx>
    3. I like Sergei's comments on TAKE and DROP:
    >I would prefer to see (SUBLIST list start end) which
    >is both generic and quite natural, and (LIST-HEAD list k)
    >to complement existing (LIST-TAIL list k).
    Perhaps for DROP, LIST-BUTLAST ala Common Lisp and LIST-BUTFIRST (or NTH-CDR).

From: Maciej Stachowiak <mstachow@xxxxxxx>
    Why not `list-head' and `list-tail' for these, I think those are more
    intuitive names. `list-ref' vs. `nth' is more an open question, why
    not have both.

I do not like LIST-HEAD and LIST-TAIL because I find the names confusing.
Does (LIST-TAIL list k) *drop* K elements or *give* you a K-element tail? With
TAKE and DROP there is no such confusion. If you *want* K elements, you TAKE
them. If you want to *remove* K elements, you DROP them.

I particularly dislike the names LIST-HEAD and LIST-TAIL. By all rights,
these names should be synonyms for CAR and CDR. The head of a list is its
car; the tail of a list is its cdr.

Matt Flatt argues that separate functions to take from the front and
back of the list gives better error checking; similarly for dropping.

This is a good point. Note that we currently have four procedures:
    take drop take! drop!
This would split each of these procedures into two, for a total of eight
procedures. We could call them
    take take/right   take! take/right!
    drop drop/right   drop! drop/right
So (DROP LIST 3) would drop the first 3 elements of LIST and
   (DROP/RIGHT LIST 3) would drop the last 3 elements of LIST.
This is good & bad. Good is that is might give better dynamic error checking,
should a client erroneously pass a negative index to the function. On the
other hand, I don't think it's a very common error. Fencepost errors are when
you confuse 0 & 1. Drifting over into negative values is much, much rarer. Bad
is that is induces namespace bloat, and there's a hidden multiplier lurking in
store: I will be proposing string and vector libs with TAKE and DROP functions
(e.g., STRING-TAKE and VECTOR-DROP), so the factor of two will hit these libs
as well.

There are two issues here:
    The actual functionality -- punting the negative-indexing for twice
        as many function bindings.
    In the event of punting negative-indexing, we must deal with the
        naming choices for the left-end and right-end variants.
	This relates to the larger naming issue discussed in topic
	"Naming: right & left variants."

[By the way, in response to questions of the source of the right-end
negative-indexing convention -- it's a popular feature in APL and J.]

-------------------------------------------------------------------------------
* Add SUBLIST ?

Several people have requested (SUBLIST lis start end). I explicitly did not
put this function in the library, since reaching into the middle of a list
and taking out a specific, fixed subsegment seems contrary to the general idea
of lists. But. I realise that many times we sleazily use lists as fixed tuples,
and in cases like this, SUBLIST can be handy (e.g., the grammatical structure
of sexp-based languages).

I do *not* think, in any event, that SUBLIST is an acceptable replacement
for TAKE and DROP. It is much clearer, to my eye, to use functions that
specifically return prefixes or suffixes than to use the general SUBLIST
function when this is what is desired. All the more so in the case of lists,
where one must do a linear-time pass over the whole list just to get the
final index to pass to SUBLIST.

So some questions: 
    - Should SUBLIST be added to the existing repertoire of TAKE & DROP funs?

    - If so, should we also add a linear-update SUBLIST! ?

    - Should I tweak the definition of SUBLIST to aid in indexing "from
      the right"?
	+ I could make the END argument optional, defaulting to the length
          of the list.
	+ I could make the END argument range over negative as well as
	  non-negative indices, indicating offsets from the right without
	  requiring the programmer to explicitly precompute the list length.
	  E.g.
	    (sublist '(a b c d e f) 2 -1) => '(c d e f)
	    (sublist '(a b c d e f) 2 -2) => '(c d e)
	    (sublist '(a b c d e f) 2 -3) => '(c d)

I'm just throwing out as wide a spectrum of sublist-related stuff as I can
think, here. Personally, I'm pretty indifferent on all of these questions,
except that I do think overall library consistency requires us to pair
SUBLIST! with SUBLIST -- neither or both, but not just the one.

-------------------------------------------------------------------------------
* Removing PROPER-LIST?

This procedure is exactly LIST? While the name "PROPER-LIST?" is slightly
clearer than "LIST?", it is not worth breaking with established use. I
am removing PROPER-LIST? from the list-lib, and leaving LIST? in.

-------------------------------------------------------------------------------
* map function termination condition

From the original proposal:
    3.When do n-ary mapping functions (MAP, MAP!, FOR-EACH, PAIR-FOR-EACH,
     APPEND-MAP, APPEND-MAP!, FILTER-MAP, MAP-IN-ORDER) terminate? 
		    1.When any list runs out? 
		    2.When the first list runs out? 
		    3.All lists must be of equal length? 
    My preferences are in the order listed. R4RS says #3.  Hence this spec
    requires #1. Any changes to this *must* happen by the end of the SRFI
    discussion period. 

The consistent feedback has been to go with definition #1. If you feel
otherwise, speak up now, otherwise I will regard this issue as closed.
Below I list two representative remarks I have received on this issue.

Donovan Kolbly
    Dylan, which has a fairly general collections mechanism, also takes
    approach #1.  Generalizing lists, a collection in Dylan is regarded as a
    mapping from keys to values.  The keys for lists and vectors are integers
    starting at zero.  n-ary mapping functions do an intersection-join on the
    keys of their arguments, and hence, for the list case, only operate on the
    common keys, ie, along the shortest list. 

From: John David Stone <stone@xxxxxxxxxxxxx>
    As soon as any of the lists is exhausted (alternative 1 in
    Shivers's list).  My second choice is Shivers's alternative 3: all lists
    must be the same length.

-------------------------------------------------------------------------------
* FIND, FIND-TAIL n-ary

"Will Fitzgerald" <fitzgerald@xxxxxxxxxxxx>
    Since ANY, EVERY, FOLDL, FOLDR, PAIR-FOLDL, PAIR-FOLDR, and LIST-INDEX can
    take multiple lists as arguments, should FIND and FIND-TAIL do the same?

I am uncomfortable with the idea of procedures whose return "arity" depends
on their call arity. I would prefer to keep FIND and FIND-TAIL simple.

-------------------------------------------------------------------------------
* alist functions in separate lib?

Separate: John David Stone <stone@xxxxxxxxxxxxx>
Together: Olin

John David Stone <stone@xxxxxxxxxxxxx>
    Yes.  The most conclusive point for me is that they take a
    different copy procedure -- that suggests that they are really a different
    data type and so deserve a separate library.

Hmm -- I still prefer to keep the alist functions in the general list lib.

-------------------------------------------------------------------------------
* FIND-TAIL applies pred to list cells or list elts?

List cells:
List elts: Olin, John David Stone <stone@xxxxxxxxxxxxx>

No one supports list cells. Good. Let's consider this issue closed.

-------------------------------------------------------------------------------
* Are the right-duplicate deletion procedures worth inclusion?

No: John David Stone <stone@xxxxxxxxxxxxx>
Yes: Olin

Do others have opinions?

-------------------------------------------------------------------------------
* lists-as-sets funs put in separate module?

Together: Olin
Separate: John David Stone <stone@xxxxxxxxxxxxx>

stone:
    "It should be kept separate.  Again the crucial argument is that
    sets are a different data type: EQUAL-AS-SETS? will not be the same as
    EQUAL?, for instance."

In my view, lists-as-sets aren't a different data type, they are a particular
use of lists. These functions are sufficiently useful to warrant being
included in the general list lib.

The list-set functions are found in SRFI-3
    http://srfi.schemers.org/srfi-3/

-------------------------------------------------------------------------------
* Naming: REMOVE / DELETE conflicts

John David Stone <stone@xxxxxxxxxxxxx>
"Sergei Egorov" <esl@xxxxxxxxxxxxxxx>

The issue is that some Schemes use DELETE to name the functions that delete
elements from lists using equality tests, and some use REMOVE. SRFI-1 has gone
with DELETE, and uses REMOVE to name the functions that filter lists with
a predicate.

There are conflicts no matter which one I choose, and solid precedent over part
of the community with the current choice:
    T, S48, MIT Scheme: delq/delv/delete/delq!/delv!/delete!
    Bigloo, Chez, MzScheme: remq/remove/remove!

CommonLisp has both REMOVE and DELETE -- the former being pure, and the
latter being destructive. This is a terrible naming convention; Scheme
has the bang suffix, which makes for a much clearer and tighter linkage.
We get no guidance here. (And CL's naming is probably why Scheme
implementations have diverged on this issue. Urk.)

Unless I hear of a good alternative name for list-lib's REMOVE, I will
keep with the current uses of DELETE and REMOVE and their derived names.
What is needed is a name to replace REMOVE that fits in with this trio:
    FILTER / PARTITION / ???
REMOVE is the best, most natural simple root I could think of. FILTER-NOT or
NKEEP or KEEP-NOT are awkward.  EXTIRPATE seems a little over-the-top.  I am
not a fan of the -IF suffix. It looks awkward to my eye; I associate IF with
conditional forms, not variables bound to procedures.

-------------------------------------------------------------------------------
* More careful specification of error cases

Matthias Felleisen <matthias@xxxxxxxxxxx>
    ...the specification for a procedure like TAKE should contain a sentence
    like "It is an error if <i> is larger than the length of <list>."

    Furthermore, I believe that libraries should go even further and specify 
    that 

      "it is an error if a procedure whose i-th parameter is specified to be a
       <list> receives an i-th argument that does not belong to the collection of
       <lists>."

    Again, this gives the implementation the freedom to delay signaling an
    error until the non-listness of the argument is discovered or not to signal
    an error or to be preemptive in checking the nature of all arguments. Of
    course, the statement should be generalized over <list> and <lists> as
    appropriate. 

A more careful specification of error cases is a good thing; I will work on
this. I have already added a great deal of argument checking to the latest
version of the reference implementation, per Matthias's prodding, and will
make another pass over the spec.

-------------------------------------------------------------------------------
* Argument order for FOLDL and FOLDR

John David Stone <stone@xxxxxxxxxxxxx> points out that the n-list case tends
to suggest 
    (f <state-value> <elt1> ... <eltn>)
rather than  
    (f <elt1> ... <eltn> <state-value>)

Good point, but I want consistency between the two functions.

state-value first: srfi-1, SML
state-value last: Haskell

-------------------------------------------------------------------------------
* Naming: right & left variants

This is a *thorny* naming issue. Many of the procedures in this list lib, and
also in the upcoming vector and string libs I will be proposing, come in
left/right pairs. I have been using an L and R suffix to name these pairs,
e.g. FOLDL and FOLDR, REDUCEL and REDUCER. This has the charm of being concise
-- as should be obvious by now, I code in 80-column buffers and do not like to
waste columns gratuitously. However, many people prefer longer names. Also,
some of the resultant names are unfortunate -- the most glaringly awkward one
is REDUCER, which really just means "REDUCE from the Right."  So my L and R
suffix convention is not without its downsides. But I like short. I like 
simple. And FOLDL, in particular, is well-established in the FP world.

As will be obvious below, this is a very important naming convention to get
right, due to its pervasiveness across multiple libraries and many, many
names. So it really takes some careful thought.

I see three alternatives to consider:

- use *no* suffix for the left operator, and a /R suffix for the right
  operator. This is based on the idea that left-to-right is the "natural"
  processing order for lists. This gives us the following list, vector and
  string bindings (assuming we split the TAKE and DROP ops into left & right
  variants):

    fold	fold/r		reduce	reduce/r
    take	take/r    	take!	take!/r
    drop	drop/r    	drop!	drop!/r
    pair-fold	pair-fold/r

    vector-take vector-take/r vector-take/shared vector-take/rshared
    vector-drop vector-drop/r vector-drop/shared vector-drop/rshared
    vector-find vector-find/r vector-skip vector-skip/r
    vector-fold vector-fold/r vector-reduce vector-reduce/r 
    vector-scan vector-scan/r

    string-fold  string-fold/r
    string-take string-take/r
    string-drop string-drop/r
    string-pad  string-pad/r
    string-trim string-trim/r
    string-index string-index/r
    string-skip string-skip/r

- use an /L suffix for the left operator, and an /R suffix for the right
  operator. This gives us the following list, vector and string bindings:

    fold/l	fold/r		reduce/l	reduce/r
    take/l	take/r    	take!/l		take!/r
    drop/l	drop/r    	drop!/l		drop!/r
    pair-fold/l	pair-fold/r

    vector-take/l vector-take/r vector-take/lshared vector-take/rshared
    vector-drop/l vector-drop/r vector-drop/lshared vector-drop/rshared
    vector-find/l vector-find/r vector-skip/l vector-skip/r
    vector-fold/l vector-fold/r vector-reduce/l vector-reduce/r 
    vector-scan/l vector-scan/r

    string-fold/l  string-fold/r
    string-take/l string-take/r
    string-drop/l string-drop/r
    string-pad/l  string-pad/r
    string-trim/l string-trim/r
    string-index/l string-index/r
    string-skip/l string-skip/r

- use a -left suffix for the left operator, and a -right suffix for the right
  operator. This gives us the following list, vector and string bindings:

    fold-left	fold-right	reduce-left	reduce-right
    take-left	take-right    	take-left!	take-right!
    drop-left	drop-right    	drop-left!	drop-right!
    pair-fold-left		pair-fold-right

    vector-take-left vector-take-right vector-take-left/shared vector-take-right/shared
    vector-drop-left vector-drop-right vector-drop-left/shared vector-drop-right/shared
    vector-find-left vector-find-right vector-skip-left vector-skip-right
    vector-fold-left vector-fold-right vector-reduce-left vector-reduce-right 
    vector-scan-left vector-scan-right

    string-fold-left	string-fold-right
    string-take-left	string-take-right
    string-drop-left	string-drop-right
    string-pad-left	string-pad-right
    string-trim-left	string-trim-right
    string-index-left	string-index-right
    string-skip-left	string-skip-right

  Ouch, I find these names painfully verbose for primitive, low-level
  operations. It makes it hard to use functional composition -- 
  (f (g (h x))) -- without drifting off the right side of the screen or
  shifting over to awkward, multiline indenting styles. Also, the actual 
  operation (fold, reduce, pad, trim) gets lost amidst all the tacked-on
  modifiers.

However, I listen attentively to the community voice.

[Let's not worry about what exactly these "vector shared" ops are; that is
a topic for another SRFI and another time. Let's focus on this issue of
left and right variant names.]

-------------------------------------------------------------------------------
* Add UNZIP1 ?

Egorov:
    UNZIP1 is missing although it no less useful than other
    procedures of the UNZIP family:
        (unzip1 '((1) (2) (3))) => (1 2 3)

This is just (MAP CAR list). But I will add the binding if there is demand for
it; it seems like a reasonable thing to do simply for consistency, to avoid
surprise. May I hear some opinions?

-------------------------------------------------------------------------------
* destructive/linear-update

Sperber has checked in verbally supporting weakening the spec for the
bang procedures to be linear-update.

Clinger claims some people claim side-effects are needed.

Lars says he himself needs guaranteed side-effects.  Lars supports having both
linear-update and guaranteed side-effect bindings. Note that this doesn't
complicate simple implementations as all -- it just means you implement the
side-effect version, and bind it to both names.

I'd like to hear more support for required-mutation.

After *much* thought on this issue -- there are many possible directions
one could choose, and all have advantages and disadvantages -- I have
just recently arrived at the following proposal that will serve Lars'
concerns, not bloat out the basic lib, and has what strikes me as a
reasonably natural and concise naming convention.

Let us proceed on the assumption that required-mutation is the rare case,
albeit one we will support. We will use a *double* bang for these names --
extra emphasis!! Really change the list!! We can then place these procedures
in a separate library, list-lib!!. Here are the procedures we'd add
   take!! drop!! 
   append!! reverse-append!!
   append-map!! map!!
   filter!! partition!! remove!! 
   del!! delq!! delv!! delete!!
   delq-duplicates!! delv-duplicates!! delete-duplicates!! del-duplicates!!
   alist-delete!! del-ass!! del-assq!! del-assv!! del-assoc!!
   reverse!!
(Again, note that in most cases, these names will be bound to the exact
same procedures to which their single-bang cousins will be bound.)

Now if someone like Lars writes code that *relies*, e.g., due to sharing, on
really performing side-effects, instead of simply *permitting* side-effects as
an optimisation, e.g., due to non-sharing, the double-bangs will draw the eye
to these semantically effectful operations. (What we are doing here is
separating side-effects as pragmatics from side-effects as semantics.)

An alternate would be to use a + suffix to indicate linear-update, and
reserve ! for required-side-effect. We could still move all the
required-side-effect procs to a segregated library.

-------------------------------------------------------------------------------
* Naming: ACONS or ALIST-CONS?

acons:
alist-cons: Lars Thomas Hansen <lth@xxxxxxxxxxx>

I am indifferent, and am happy to go with Lars' suggestion. Could we have some
more votes?

-------------------------------------------------------------------------------
* Naming: PAIR-frob prefix vs frob-TAIL suffix 

E.g., pair-for-each or for-each-tail? pair-fold or foldl-tail?

jpiitula & egorov

egorov:
    8) I prefer having TAIL- prefix for procedures working with
    consecutive cdrs of a list; PAIR- prefix does not have this "CDR"
    sound (PAIR-FOR-EACH may be a better name for tree browsing
    procedure)

These procedures work directly with the pairs or cons cells that compose the
list, hence the PAIR- lexeme.  I don't like the TAIL- convention, as the
function doesn't operate just on the tail of the argument list, but also on
the list itself.  Also, these functions *don't* operate on the tail of the
list that is the empty list () -- which is certainly a tail. However, my
preference for PAIR- is not a strong one.

I would like to hear other opinions on this name choice.

-------------------------------------------------------------------------------
* MAKE-LIST's default fill argument

jpiitula:
    I think that MAKE-LIST should not allow the 'fill'
    argument to be left out; at least, the default value
    should be unspecified as in MAKE-VECTOR and MAKE-STRING
    (choice of #f seems a little random to me).

I have changed MAKE-LIST's spec so that the default fill value is unspecified
as suggested by jpiitula & egorov.

Dissenters should speak up; but I don't think there will be any.

-------------------------------------------------------------------------------
* Naming: CONS* or LIST*

General consensus is that CONS* is a better name. I have changed the name
accordingly.

-------------------------------------------------------------------------------
* Naming: APPEND-REVERSE{!} or REVERSE-APPEND{!}

REVERSE-APPEND is the current name.
T & S48 use APPEND-REVERSE
Common Lisp: REVAPPEND

Egorov votes for APPEND-REVERSE, and points out it visually matches up
with the definition (append (reverse x) y). It is also consistent with
the current Scheme uses I've found (T & Scheme 48). I will make the
change.

Are there any other runtimes or libs that export these procedures,
and, if so, what are the chosen names?

-------------------------------------------------------------------------------
* Argument order of = equivalence predicates

Egorov
    I would also left unspecified the behavior of procedures accepting
    equivalence predicates [=] when given non-commutative procedures; when in
    doubt, one can always use -IF variants.

This is an excellent point. However, I suggest it would be more useful to
address this issue by specifying more precisely how the = predicate is used.
Spelling out how the equivalence proc is applied seems more useful to me, and
would cost nothing. How do others feel about this? I can add language to the
definitions saying that the comparison is made in this form
    (= key-param elet)
Here's the extra language:

For DEL, DEL!
    The = procedure is an equality predicate that is used to compare
    the elements Ei of LIST to the key X in this way:
	(= X Ei)
    The = predicate will be used to compare each element of LIST exactly
    once; the order in which it is applied to the various Ei is not specified.
    Thus, one can reliably remove all the numbers less than five from a list
    with
	(del < 5 list)

For DEL-DUPLICATES DEL-DUPLICATES!
    The = procedure is an equality predicate that is used to compare
    the elements of LIST. If X comes before Y in LIST, then the comparison
    is performed 
	(= X Y)
    The = predicate will be used to compare each pair of elements in LIST
    exactly once; the order in which it is applied to the various pairs is not
    specified.

For MEM
    The = procedure is an equality predicate that is used to compare
    the elements Ei of LIST to the key X in this way:
	(= X Ei)
    The = predicate will be used to compare each element of LIST no more
    than once.

For ASS
    The = procedure is an equality predicate that is used to compare
    the element keys Ki of ALIST's entries to the search-key X in this way:
	(= X Ki)
    The = predicate will be used to compare each key of ALIST no more
    than once. Thus one can reliably find the first entry of ALIST whose
    key is less than five with
	(ass < 5 ALIST)
    
For ALIST-DELETE DEL-ASS ALIST-DELETE! DEL-ASS!
    The = procedure is an equality predicate that is used to compare
    the elements keys Ki of ALIST's entries to the key X in this way:
	(= X Ki)
    The = predicate will be used to compare each element key of ALIST exactly
    once; the order in which it is applied to the various Ki is not specified.
    Thus, one can reliably remove all entries of ALIST whose key is less than
    five with
	(del < 5 list)