by Olin Shivers
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lset=
.)R5RS Scheme has an impoverished set of listprocessing utilities, which is a problem for authors of portable code. This SRFI proposes a coherent and comprehensive set of listprocessing procedures; it is accompanied by a reference implementation of the spec. The reference implementation is
The set of basic list and pair operations provided by R4RS/R5RS Scheme is far from satisfactory. Because this set is so small and basic, most implementations provide additional utilities, such as a listfiltering function, or a "left fold" operator, and so forth. But, of course, this introduces incompatibilities  different Scheme implementations provide different sets of procedures.
I have designed a fullfeatured library of procedures for list processing. While putting this library together, I checked as many Schemes as I could get my hands on. (I have a fair amount of experience with several of these already.) I missed Chez  no online manual that I can find  but I hit most of the other big, fullfeatured Schemes. The complete list of listprocessing systems I checked is:
As a result, the library I am proposing is fairly rich.
Following this initial design phase, this library went through several months of discussion on the SRFI mailing lists, and was altered in light of the ideas and suggestions put forth during this discussion.
In parallel with designing this API, I have also written a reference implementation. I have placed this source on the Net with an unencumbered, "open" copyright. A few notes about the reference implementation:
error
procedure;
values
and a simple receive
macro for producing
and consuming multiple return values;
:optional
and letoptionals
macros for optional
argument parsing and defaulting;
checkarg
procedure for argument checking.
It is written for efficiency. Fast paths are provided for common
cases. Sideeffecting procedures such as filter!
avoid unnecessary,
redundant setcdr!
s which would thrash a generational GC's write barrier
and the store buffers of fast processors. Functions reuse longest common
tails from input parameters to construct their results where
possible. Constantspace iterations are used in preference to recursions;
local recursions are used in preference to consing temporary intermediate
data structures.
This is not to say that the implementation can't be tuned up for a specific Scheme implementation. There are notes in comments addressing ways implementors can tune the reference implementation for performance.
In short, I've written the reference implementation to make it as painless as possible for an implementor  or a regular programmer  to adopt this library and get good results with it.
Here is a short list of the procedures provided by the listlib package. R5RS procedures are shown in bold; extended R5RS procedures, in bold italic.
cons list xcons cons* makelist listtabulate listcopy circularlist iota
pair? null? properlist? circularlist? dottedlist? notpair? nulllist? list=
car cdr ... cddadr cddddr listref first second third fourth fifth sixth seventh eighth ninth tenth car+cdr take drop takeright dropright take! dropright! splitat splitat! last lastpair
length length+ append concatenate reverse append! concatenate! reverse! appendreverse appendreverse! zip unzip1 unzip2 unzip3 unzip4 unzip5 count
map foreach fold unfold pairfold reduce foldright unfoldright pairfoldright reduceright appendmap appendmap! map! pairforeach filtermap mapinorder
filter partition remove filter! partition! remove!
member memq memv find findtail any every listindex takewhile dropwhile takewhile! span break span! break!
delete deleteduplicates delete! deleteduplicates!
assoc assq assv alistcons alistcopy alistdelete alistdelete!
lset<= lset= lsetadjoin lsetunion lsetunion! lsetintersection lsetintersection! lsetdifference lsetdifference! lsetxor lsetxor! lsetdiff+intersection lsetdiff+intersection!
setcar! setcdr!
Four R4RS/R5RS listprocessing procedures are extended by this library in backwardscompatible ways:
map foreach
 (Extended to take lists of unequal length) 
member assoc
 (Extended to take an optional comparison procedure.) 
The following R4RS/R5RS list and pairprocessing procedures are also part of listlib's exports, as defined by the R5RS:
cons pair? null? car cdr ... cdddar cddddr setcar! setcdr! list append reverse length listref memq memv assq assv
The remaining two R4RS/R5RS listprocessing procedures are not part of this library:
listtail
 (renamed drop )

list?
 (see properlist? ,
circularlist? and
dottedlist? )

A set of general criteria guided the design of this library.
I don't require "destructive" (what I call "linear update") procedures to alter and recycle cons cells from the argument lists. They are allowed to, but not required to. (And the reference implementations I have written do recycle the argument lists.)
Listfiltering procedures such as filter
or delete
do not disorder
lists. Elements appear in the answer list in the same order as they appear in
the argument list. This constrains implementation, but seems like a desirable
feature, since in many uses of lists, order matters. (In particular,
disordering an alist is definitely a bad idea.)
Contrariwise, although the reference implementations of the listfiltering procedures share longest common tails between argument and answer lists, it not is part of the spec.
Because lists are an inherently sequential data structure (unlike, say,
vectors), listinspection functions such as find
, findtail
, foreach
, any
and every
commit to a lefttoright traversal order of their argument list.
However, constructor functions, such as
and the mapping
procedures (listtabulate
appendmap
, appendmap!
, map!
, pairforeach
, filtermap
,
mapinorder
), do not specify the dynamic order in which their procedural
argument is applied to its various values.
Predicates return useful true values wherever possible. Thus any
must return
the true value produced by its predicate, and every
returns the final true
value produced by applying its predicate argument to the last element of its
argument list.
Functionality is provided both in pure and linearupdate (potentially destructive) forms wherever this makes sense.
No special status accorded Scheme's builtin equality functions.
Any functionality provided in terms of eq?
, eqv?
, equal?
is also
available using a clientprovided equality function.
Proper design counts for more than backwards compatibility, but I have tried, ceteris paribus, to be as backwardscompatible as possible with existing listprocessing libraries, in order to facilitate porting old code to run as a client of the procedures in this library. Name choices and semantics are, for the most part, in agreement with existing practice in many current Scheme systems. I have indicated some incompatibilities in the following text.
These procedures are not "sequence generic"  i.e., procedures that operate on either vectors and lists. They are listspecific. I prefer to keep the library simple and focussed.
I have named these procedures without a qualifying initial "list" lexeme,
which is in keeping with the existing set of listprocessing utilities in
Scheme.
I follow the general Scheme convention (vectorlength, stringref) of
placing the typename before the action when naming procedures  so
we have listcopy
and pairforeach
rather than the perhaps
more fluid, but less consistent, copylist
or foreachpair
.
I have generally followed a regular and consistent naming scheme, composing procedure names from a set of basic lexemes.
Many procedures in this library have "pure" and "linear update" variants. A
"pure" procedure has no sideeffects, and in particular does not alter its
arguments in any way. A "linear update" procedure is allowed  but not
required  to sideeffect its arguments in order to construct its
result. "Linear update" procedures are typically given names ending with an
exclamation point. So, for example, (append! list1 list2)
is allowed to
construct its result by simply using setcdr!
to set the cdr of the last pair
of list_{1} to point to list_{2}, and then returning list_{1} (unless list_{1} is the
empty list, in which case it would simply return list_{2}). However, append!
may
also elect to perform a pure append operation  this is a legal definition
of append!
:
(define append! append)
This is why we do not call these procedures "destructive"  because they aren't required to be destructive. They are potentially destructive.
What this means is that you may only apply linearupdate procedures to values that you know are "dead"  values that will never be used again in your program. This must be so, since you can't rely on the value passed to a linearupdate procedure after that procedure has been called. It might be unchanged; it might be altered.
The "linear" in "linear update" doesn't mean "linear time" or "linear space" or any sort of multipleofn kind of meaning. It's a fancy term that type theorists and pure functional programmers use to describe systems where you are only allowed to have exactly one reference to each variable. This provides a guarantee that the value bound to a variable is bound to no other variable. So when you use a variable in a variable reference, you "use it up." Knowing that no one else has a pointer to that value means the a system primitive is free to sideeffect its arguments to produce what is, observationally, a purefunctional result.
In the context of this library, "linear update" means you, the programmer, know there are no other live references to the value passed to the procedure  after passing the value to one of these procedures, the value of the old pointer is indeterminate. Basically, you are licensing the Scheme implementation to alter the data structure if it feels like it  you have declared you don't care either way.
You get no help from Scheme in checking that the values you claim are "linear" really are. So you better get it right. Or play it safe and use the non! procedures  it doesn't do any good to compute quickly if you get the wrong answer.
Why go to all this trouble to define the notion of "linear update" and use it in a procedure spec, instead of the more common notion of a "destructive" operation? First, note that destructive listprocessing procedures are almost always used in a linearupdate fashion. This is in part required by the special case of operating upon the empty list, which can't be sideeffected. This means that destructive operators are not pure sideeffects  they have to return a result. Second, note that code written using linearupdate operators can be trivially ported to a pure, functional subset of Scheme by simply providing pure implementations of the linearupdate operators. Finally, requiring destructive sideeffects ruins opportunities to parallelise these operations  and the places where one has taken the trouble to spell out destructive operations are usually exactly the code one would want a parallelising compiler to parallelise: the efficiencycritical kernels of the algorithm. Linearupdate operations are easily parallelised. Going with a linearupdate spec doesn't close off these valuable alternative implementation techniques. This list library is intended as a set of lowlevel, basic operators, so we don't want to exclude these possible implementations.
The linearupdate procedures in this library are
take! dropright! splitat!
append! concatenate! reverse! appendreverse!
appendmap! map!
filter! partition! remove!
takewhile! span! break!
delete! alistdelete! deleteduplicates!
lsetadjoin! lsetunion! lsetintersection!
lsetdifference! lsetxor! lsetdiff+intersection!
Scheme does not properly have a list type, just as C does not have a string type. Rather, Scheme has a binarytuple type, from which one can build binary trees. There is an interpretation of Scheme values that allows one to treat these trees as lists. Further complications ensue from the fact that Scheme allows sideeffects to these tuples, raising the possibility of lists of unbounded length, and trees of unbounded depth (that is, circular data structures).
However, there is a simple view of the world of Scheme values that considers every value to be a list of some sort. that is, every value is either
(a b c)
()
(32)
(a b c . d)
(x . y)
42
george
Note that the zerolength dotted lists are simply all the nonnull, nonpair values.
This view is captured by the predicates properlist?
, dottedlist?
, and
circularlist?
. Listlib users should note that dotted lists are not commonly
used, and are considered by many Scheme programmers to be an ugly artifact of
Scheme's lack of a true list type. However, dotted lists do play a noticeable
role in the syntax of Scheme, in the "rest" parameters used by nary
lambdas: (lambda (x y . rest) ...)
.
Dotted lists are not fully supported by listlib. Most procedures are defined only on proper lists  that is, finite, nilterminated lists. The procedures that will also handle circular or dotted lists are specifically marked. While this design decision restricts the domain of possible arguments one can pass to these procedures, it has the benefit of allowing the procedures to catch the error cases where programmers inadvertently pass scalar values to a list procedure by accident, e.g., by switching the arguments to a procedure call.
Note that statements of the form "it is an error" merely mean "don't
do that." They are not a guarantee that a conforming implementation will
"catch" such improper use by, for example, raising some kind of exception.
Regrettably, R5RS Scheme requires no firmer guarantee even for basic operators such
as car
and cdr
, so there's little point in requiring these procedures to do
more. Here is the relevant section of the R5RS:
When speaking of an error situation, this report uses the phrase "an error is signalled" to indicate that implementations must detect and report the error. If such wording does not appear in the discussion of an error, then implementations are not required to detect or report the error, though they are encouraged to do so. An error situation that implementations are not required to detect is usually referred to simply as "an error."
For example, it is an error for a procedure to be passed an argument that the procedure is not explicitly specified to handle, even though such domain errors are seldom mentioned in this report. Implementations may extend a procedure's domain of definition to include such arguments.
The following items are not in this library:
They should have their own SRFI specs.
In a Scheme system that has a module or package system, these procedures should be contained in a module named "listlib".
The templates given below obey the following conventions for procedure formals:
list  A proper (finite, nilterminated) list 

clist  A proper or circular list 
flist  A finite (proper or dotted) list 
pair  A pair 
x, y, d, a  Any value 
object, value  Any value 
n, i  A natural number (an integer >= 0) 
proc  A procedure 
pred  A procedure whose return value is treated as a boolean 
=  A boolean procedure taking two arguments 
It is an error to pass a circular or dotted list to a procedure not defined to accept such an argument.
cons
a d > pair
[R5RS]
The primitive constructor. Returns a newly allocated pair whose car is
a and whose cdr is d.
The pair is guaranteed to be different (in the sense of eqv?
)
from every existing object.
(cons 'a '()) => (a) (cons '(a) '(b c d)) => ((a) b c d) (cons "a" '(b c)) => ("a" b c) (cons 'a 3) => (a . 3) (cons '(a b) 'c) => ((a b) . c)
list
object ... > list
[R5RS] Returns a newly allocated list of its arguments.
(list 'a (+ 3 4) 'c) => (a 7 c) (list) => ()
xcons
d a > pair
(lambda (d a) (cons a d))
Of utility only as a value to be conveniently passed to higherorder procedures.
(xcons '(b c) 'a) => (a b c)
The name stands for "eXchanged CONS."
cons*
elt_{1} elt_{2} ... > object
Like list
,
but the last argument provides the tail of the constructed list,
returning
(cons elt_{1} (cons elt_{2} (cons ... elt_{n})))
This function is called list*
in Common Lisp and about
half of the Schemes that provide it,
and cons*
in the other half.
(cons* 1 2 3 4) => (1 2 3 . 4) (cons* 1) => 1
makelist
n [fill] > list
Returns an nelement list, whose elements are all the value fill. If the fill argument is not given, the elements of the list may be arbitrary values.
(makelist 4 'c) => (c c c c)
listtabulate
n initproc > list
Returns an nelement list. Element i of the list, where 0 <= i < n,
is produced by (initproc i)
. No guarantee is made about the dynamic
order in which initproc is applied to these indices.
(listtabulate 4 values) => (0 1 2 3)
listcopy
flist > flist
Copies the spine of the argument.
circularlist
elt_{1} elt_{2} ... > list
Constructs a circular list of the elements.
(circularlist 'z 'q) => (z q z q z q ...)
iota
count [start step] > list
Returns a list containing the elements
(start start+step ... start+(count1)*step)
The start and step parameters default to 0 and 1, respectively. This procedure takes its name from the APL primitive.
(iota 5) => (0 1 2 3 4) (iota 5 0 0.1) => (0 0.1 0.2 0.3 0.4)
Note: the predicates properlist?
, circularlist?
, and dottedlist?
partition the entire universe of Scheme values.
properlist?
x > boolean
Returns true iff x is a proper list  a finite, nilterminated list.
More carefully: The empty list is a proper list. A pair whose cdr is a proper list is also a proper list:
<properlist> ::= () (Empty proper list)  (cons <x> <properlist>) (Properlist pair)
Note that this definition rules out circular lists. This function is required to detect this case and return false.
Nilterminated lists are called "proper" lists by R5RS and Common Lisp. The opposite of proper is improper.
R5RS binds this function to the variable list?
.
(not (properlist? x)) = (or (dottedlist? x) (circularlist? x))
circularlist?
x > boolean
True if x is a circular list. A circular list is a value such that for every n >= 0, cdr^{n}(x) is a pair.
Terminology: The opposite of circular is finite.
(not (circularlist? x)) = (or (properlist? x) (dottedlist? x))
dottedlist?
x > boolean
True if x is a finite, nonnilterminated list. That is, there exists an n >= 0 such that cdr^{n}(x) is neither a pair nor (). This includes nonpair, non() values (e.g. symbols, numbers), which are considered to be dotted lists of length 0.
(not (dottedlist? x)) = (or (properlist? x) (circularlist? x))
pair?
object > boolean
[R5RS] Returns #t if object is a pair; otherwise, #f.
(pair? '(a . b)) => #t (pair? '(a b c)) => #t (pair? '()) => #f (pair? '#(a b)) => #f (pair? 7) => #f (pair? 'a) => #f
null?
object > boolean
[R5RS] Returns #t if object is the empty list; otherwise, #f.
nulllist?
list > boolean
List is a proper or circular list. This procedure returns true if the argument is the empty list (), and false otherwise. It is an error to pass this procedure a value which is not a proper or circular list.
This procedure is recommended as the termination condition for listprocessing procedures that are not defined on dotted lists.
notpair?
x > boolean
(lambda (x) (not (pair? x)))
Provided as a procedure as it can be useful as the termination condition for listprocessing procedures that wish to handle all finite lists, both proper and dotted.
list=
elt= list_{1} ... > boolean
Determines list equality, given an elementequality procedure.
Proper list A equals proper list B
if they are of the same length,
and their corresponding elements are equal,
as determined by elt=.
If the elementcomparison procedure's first argument is
from list_{i},
then its second argument is from list_{i+1},
i.e. it is always called as
(elt= a b)
for a an element of list A,
and b an element of list B.
In the nary case,
every list_{i} is compared to
list_{i+1}
(as opposed, for example, to comparing
list_{1} to every list_{i},
for i>1).
If there are no list arguments at all,
list=
simply returns true.
It is an error to apply list=
to anything except proper lists.
While
implementations may choose to extend it to circular lists, note that it
cannot reasonably be extended to dotted lists, as it provides no way to
specify an equality procedure for comparing the list terminators.
Note that the dynamic order in which the elt= procedure is
applied to pairs of elements is not specified.
For example, if list=
is applied
to three lists, A, B, and C,
it may first completely compare A to B,
then compare B to C,
or it may compare the first elements of A and B,
then the first elements of B and C,
then the second elements of A and B, and so forth.
The equality procedure must be consistent with eq?
.
That is, it must be the case that
(eq? x y)
=> (elt= x y)
.
Note that this implies that two lists which are eq?
are always list=, as well; implementations may exploit this
fact to "shortcut" the elementbyelement comparisons.
(list= eq?) => #t ; Trivial cases (list= eq? '(a)) => #t
car
pair > value
cdr
pair > value
[R5RS] These functions return the contents of the car and cdr field of their argument, respectively. Note that it is an error to apply them to the empty list.
(car '(a b c)) => a (cdr '(a b c)) => (b c) (car '((a) b c d)) => (a) (cdr '((a) b c d)) => (b c d) (car '(1 . 2)) => 1 (cdr '(1 . 2)) => 2 (car '()) => *error* (cdr '()) => *error*
caar
pair > value
cadr
pair > value
:
cddadr
pair > value
cdddar
pair > value
cddddr
pair > value
[R5RS]
These procedures are compositions of car
and cdr
,
where for example caddr
could be defined by
(define caddr (lambda (x) (car (cdr (cdr x))))).
Arbitrary compositions, up to four deep, are provided. There are twentyeight of these procedures in all.
listref
clist i > value
[R5RS]
Returns the i^{th} element of clist.
(This is the same as the car of
(drop clist i)
.)
It is an error if i >= n,
where n is the length of clist.
(listref '(a b c d) 2) => c
first
pair > object
second
pair > object
third
pair > object
fourth
pair > object
fifth
pair > object
sixth
pair > object
seventh
pair > object
eighth
pair > object
ninth
pair > object
tenth
pair > object
car
, cadr
, caddr
, ...
(third '(a b c d e)) => c
car+cdr
pair > [x y]
(lambda (p) (values (car p) (cdr p)))This can, of course, be implemented more efficiently by a compiler.
take
x i > list
drop
x i > object
take
returns the first i elements of list x.drop
returns all but the first i elements of list x.
(take '(a b c d e) 2) => (a b) (drop '(a b c d e) 2) => (c d e)x may be any value  a proper, circular, or dotted list:
(take '(1 2 3 . d) 2) => (1 2) (drop '(1 2 3 . d) 2) => (3 . d) (take '(1 2 3 . d) 3) => (1 2 3) (drop '(1 2 3 . d) 3) => dFor a legal i,
take
and drop
partition the list in a manner which
can be inverted with append
:
(append (take x i) (drop x i)) = x
drop
is exactly equivalent to performing i cdr operations on x;
the returned value shares a common tail with x.
If the argument is a list of nonzero length, take
is guaranteed to
return a freshlyallocated list, even in the case where the entire
list is taken, e.g. (take lis (length lis))
.
takeright
flist i > object
dropright
flist i > list
takeright
returns the last i elements of flist.
dropright
returns all but the last i elements of flist.
(takeright '(a b c d e) 2) => (d e) (dropright '(a b c d e) 2) => (a b c)
The returned list may share a common tail with the argument list.
flist may be any finite list, either proper or dotted:
(takeright '(1 2 3 . d) 2) => (2 3 . d) (dropright '(1 2 3 . d) 2) => (1) (takeright '(1 2 3 . d) 0) => d (dropright '(1 2 3 . d) 0) => (1 2 3)
For a legal i, takeright
and dropright
partition the list in a manner
which can be inverted with append
:
(append (take flist i) (drop flist i)) = flist
takeright
's return value is guaranteed to share a common tail with flist.
If the argument is a list of nonzero length, dropright
is guaranteed to
return a freshlyallocated list, even in the case where nothing is
dropped, e.g. (dropright lis 0)
.
take!
x i > list
dropright!
flist i > list
take!
and dropright!
are "linearupdate" variants of take
and
dropright
: the procedure is allowed, but not required, to alter the
argument list to produce the result.
If x is circular, take!
may return a shorterthanexpected list:
(take! (circularlist 1 3 5) 8) => (1 3) (take! (circularlist 1 3 5) 8) => (1 3 5 1 3 5 1 3)
splitat
x i > [list object]
splitat!
x i > [list object]
splitat
splits the list x
at index i, returning a list of the
first i elements, and the remaining tail. It is equivalent
to
(values (take x i) (drop x i))
splitat!
is the linearupdate variant. It is allowed, but not
required, to alter the argument list to produce the result.
(splitat '(a b c d e f g h) 3) => (a b c) (d e f g h)
last
pair > object
lastpair
pair > pair
last
returns the last element of the nonempty,
finite list pair.
lastpair
returns the last pair in the nonempty,
finite list pair.
(last '(a b c)) => c (lastpair '(a b c)) => (c)
length
list > integer
length+
clist > integer or #f
Both length
and length+
return the length of the argument.
It is an error to pass a value to length
which is not a proper
list (finite and nilterminated). In particular, this means an
implementation may diverge or signal an error when length
is
applied to a circular list.
length+
, on the other hand, returns #F
when applied to a circular
list.
The length of a proper list is a nonnegative integer n such that cdr
applied n times to the list produces the empty list.
append
list_{1} ... > list
append!
list_{1} ... > list
[R5RS]
append
returns a list consisting of the elements
of list_{1}
followed by the elements of the other list parameters.
(append '(x) '(y)) => (x y) (append '(a) '(b c d)) => (a b c d) (append '(a (b)) '((c))) => (a (b) (c))
The resulting list is always newly allocated, except that it shares structure with the final list_{i} argument. This last argument may be any value at all; an improper list results if it is not a proper list. All other arguments must be proper lists.
(append '(a b) '(c . d)) => (a b c . d) (append '() 'a) => a (append '(x y)) => (x y) (append) => ()
append!
is the "linearupdate" variant of append
 it is allowed, but not required, to alter cons cells in the argument
lists to construct the result list.
The last argument is never altered; the result
list shares structure with this parameter.
concatenate
listoflists > value
concatenate!
listoflists > value
These functions append the elements of their argument together.
That is, concatenate
returns
(apply append listoflists)
or, equivalently,
(reduceright append '() listoflists)
concatenate!
is the linearupdate variant, defined in
terms of append!
instead of append
.
Note that some Scheme implementations do not support passing more than a
certain number (e.g., 64) of arguments to an nary procedure.
In these implementations, the (apply append ...)
idiom
would fail when applied to long lists,
but concatenate
would continue to function properly.
As with append
and append!
,
the last element of the input list may be any value at all.
reverse
list > list
reverse!
list > list
[R5RS]
reverse
returns a newly allocated list consisting of
the elements of list in reverse order.
(reverse '(a b c)) => (c b a) (reverse '(a (b c) d (e (f)))) => ((e (f)) d (b c) a)
reverse!
is the linearupdate variant of reverse
.
It is permitted, but not required, to alter the argument's cons cells
to produce the reversed list.
appendreverse
revhead tail > list
appendreverse!
revhead tail > list
appendreverse
returns
(append (reverse revhead) tail)
.
It is provided because it is a common operation  a common
listprocessing style calls for this exact operation to transfer values
accumulated in reverse order onto the front of another list, and because
the implementation is significantly more efficient than the simple
composition it replaces. (But note that this pattern of iterative
computation followed by a reverse can frequently be rewritten as a
recursion, dispensing with the reverse
and appendreverse
steps, and
shifting temporary, intermediate storage from the heap to the stack,
which is typically a win for reasons of cache locality and eager storage
reclamation.)
appendreverse!
is just the linearupdate variant  it is allowed, but
not required, to alter revhead's cons cells to construct the result.
zip
clist_{1} clist_{2} ... > list
(lambda lists (apply map list lists))
If zip
is passed n lists, it returns a list as long as the shortest
of these lists, each element of which is an nelement list comprised
of the corresponding elements from the parameter lists.
(zip '(one two three) '(1 2 3) '(odd even odd even odd even odd even)) => ((one 1 odd) (two 2 even) (three 3 odd)) (zip '(1 2 3)) => ((1) (2) (3))
At least one of the argument lists must be finite:
(zip '(3 1 4 1) (circularlist #f #t)) => ((3 #f) (1 #t) (4 #f) (1 #t))
unzip1
list > list
unzip2
list > [list list]
unzip3
list > [list list list]
unzip4
list > [list list list list]
unzip5
list > [list list list list list]
unzip1
takes a list of lists,
where every list must contain at least one element,
and returns a list containing the initial element of each such list.
That is, it returns (map car lists)
.
unzip2
takes a list of lists, where every list must contain at least
two elements, and returns two values: a list of the first elements,
and a list of the second elements. unzip3
does the same for the first
three elements of the lists, and so forth.
(unzip2 '((1 one) (2 two) (3 three))) => (1 2 3) (one two three)
count
pred clist_{1} clist_{2} > integer
pred is a procedure taking as many arguments as there
are lists and returning a single value. It is applied
elementwise to the elements of the lists, and a count is
tallied of the number of elements that produce a true value. This count
is returned. count
is "iterative" in that it is guaranteed
to apply pred to the list elements in a
lefttoright order.
The counting stops when the shortest list expires.
(count even? '(3 1 4 1 5 9 2 5 6)) => 3 (count < '(1 2 4 8) '(2 4 6 8 10 12 14 16)) => 3
At least one of the argument lists must be finite:
(count < '(3 1 4 1) (circularlist 1 10)) => 2
fold
kons knil clist_{1} clist_{2} ... > value
The fundamental list iterator.
First, consider the single listparameter case. If clist_{1} = (e_{1} e_{2} ... e_{n}), then this procedure returns
(kons e_{n} ... (kons e_{2} (kons e_{1} knil)) ... )
That is, it obeys the (tail) recursion
(fold kons knil lis) = (fold kons (kons (car lis) knil) (cdr lis)) (fold kons knil '()) = knil
Examples:
(fold + 0 lis) ; Add up the elements of LIS. (fold cons '() lis) ; Reverse LIS. (fold cons tail revhead) ; See APPENDREVERSE. ;; How many symbols in LIS? (fold (lambda (x count) (if (symbol? x) (+ count 1) count)) 0 lis) ;; Length of the longest string in LIS: (fold (lambda (s maxlen) (max maxlen (stringlength s))) 0 lis)
If n list arguments are provided, then the kons function must take n+1 parameters: one element from each list, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest list runs out of values:
(fold cons* '() '(a b c) '(1 2 3 4 5)) => (c 3 b 2 a 1)
At least one of the list arguments must be finite.
foldright
kons knil clist_{1} clist_{2} ... > value
The fundamental list recursion operator.
First, consider the single listparameter case. If clist_{1} = (e_{1} e_{2} ... e_{n})
,
then this procedure returns
(kons e_{1} (kons e_{2} ... (kons e_{n} knil)))
That is, it obeys the recursion
(foldright kons knil lis) = (kons (car lis) (foldright kons knil (cdr lis))) (foldright kons knil '()) = knil
Examples:
(foldright cons '() lis) ; Copy LIS. ;; Filter the even numbers out of LIS. (foldright (lambda (x l) (if (even? x) (cons x l) l)) '() lis))
If n list arguments are provided, then the kons function must take n+1 parameters: one element from each list, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest list runs out of values:
(foldright cons* '() '(a b c) '(1 2 3 4 5)) => (a 1 b 2 c 3)
At least one of the list arguments must be finite.
pairfold
kons knil clist_{1} clist_{2} ... > value
Analogous to fold
, but kons is applied to successive sublists of the
lists, rather than successive elements  that is, kons is applied to the
pairs making up the lists, giving this (tail) recursion:
(pairfold kons knil lis) = (let ((tail (cdr lis)))
(pairfold kons (kons lis knil) tail))
(pairfold kons knil '()
) = knil
For finite lists, the kons function may reliably apply
setcdr!
to the pairs it is given
without altering the sequence of execution.
Example:
;;; Destructively reverse a list. (pairfold (lambda (pair tail) (setcdr! pair tail) pair) '() lis)
At least one of the list arguments must be finite.
pairfoldright
kons knil clist_{1} clist_{2} ... > value
Holds the same relationship with foldright
that pairfold
holds with fold
.
Obeys the recursion
(pairfoldright kons knil lis) =
(kons lis (pairfoldright kons knil (cdr lis)))
(pairfoldright kons knil '()
) = knil
Example:
(pairfoldright cons '() '(a b c)) => ((a b c) (b c) (c))
At least one of the list arguments must be finite.
reduce
f ridentity list > value
reduce
is a variant of fold
.
ridentity should be a "right identity" of the procedure f  that is, for any value x acceptable to f,
(f x ridentity) = x
reduce
has the following definition:
(fold f (car list) (cdr list))
.
...in other words, we compute
(fold f ridentity list)
.
Note that ridentity is used only in the emptylist case.
You typically use reduce
when applying f is expensive and you'd
like to avoid the extra application incurred when fold
applies
f to the head of list and the identity value,
redundantly producing the same value passed in to f.
For example, if f involves searching a file directory or
performing a database query, this can be significant.
In general, however, fold
is useful in many contexts where reduce
is not
(consider the examples given in the fold
definition  only one of the
five folds uses a function with a right identity.
The other four may not be performed with reduce
).
Note: MIT Scheme and Haskell flip F's arg order for their reduce
and
fold
functions.
;; Take the max of a list of nonnegative integers. (reduce max 0 nums) ; i.e., (apply max 0 nums)
reduceright
f ridentity list > value
reduceright
is the foldright variant of reduce
.
It obeys the following definition:
(reduceright f ridentity '()) = ridentity (reduceright f ridentity '(e_{1})) = (f e_{1} ridentity) = e_{1} (reduceright f ridentity '(e_{1} e_{2} ...)) = (f e_{1} (reduce f ridentity (e_{2} ...)))
...in other words, we compute
(foldright f ridentity list)
.
;; Append a bunch of lists together. ;; I.e., (apply append listoflists) (reduceright append '() listoflists)
unfold
p f g seed [tailgen] > list
unfold
is best described by its basic recursion:
(unfold p f g seed) = (if (p seed) (tailgen seed) (cons (f seed) (unfold p f g (g seed))))
(lambda (x) '())
In other words, we use g to generate a sequence of seed values
These seed values are mapped to list elements by f, producing the elements of the result list in a lefttoright order. P says when to stop.
unfold
is the fundamental recursive list constructor,
just as foldright
is
the fundamental recursive list consumer.
While unfold
may seem a bit abstract
to novice functional programmers, it can be used in a number of ways:
;; List of squares: 1^2 ... 10^2 (unfold (lambda (x) (> x 10)) (lambda (x) (* x x)) (lambda (x) (+ x 1)) 1) (unfold nulllist? car cdr lis) ; Copy a proper list. ;; Read current input port into a list of values. (unfold eofobject? values (lambda (x) (read)) (read)) ;; Copy a possibly nonproper list: (unfold notpair? car cdr lis values) ;; Append HEAD onto TAIL: (unfold nulllist? car cdr head (lambda (x) tail))
Interested functional programmers may enjoy noting that
foldright
and unfold
are in some sense inverses.
That is, given operations knull?, kar,
kdr, kons, and knil satisfying
(kons (kar x) (kdr x))
= x
and
(knull? knil)
= #t
then
(foldright kons knil (unfold knull? kar kdr x))
= x
and
(unfold knull? kar kdr (foldright kons knil x))
= x.
This combinator sometimes is called an "anamorphism;" when an explicit tailgen procedure is supplied, it is called an "apomorphism."
unfoldright
p f g seed [tail] > list
unfoldright
constructs a list with the following loop:
(let lp ((seed seed) (lis tail)) (if (p seed) lis (lp (g seed) (cons (f seed) lis))))
'()
.In other words, we use g to generate a sequence of seed values
These seed values are mapped to list elements by f, producing the elements of the result list in a righttoleft order. P says when to stop.
unfoldright
is the fundamental iterative list constructor,
just as fold
is the
fundamental iterative list consumer.
While unfoldright
may seem a bit abstract
to novice functional programmers, it can be used in a number of ways:
;; List of squares: 1^2 ... 10^2 (unfoldright zero? (lambda (x) (* x x)) (lambda (x) ( x 1)) 10) ;; Reverse a proper list. (unfoldright nulllist? car cdr lis) ;; Read current input port into a list of values. (unfoldright eofobject? values (lambda (x) (read)) (read)) ;; (appendreverse revhead tail) (unfoldright nulllist? car cdr revhead tail)
Interested functional programmers may enjoy noting that
fold
and unfoldright
are in some sense inverses.
That is, given operations knull?, kar,
kdr, kons, and knil satisfying
(kons (kar x) (kdr x))
= x
and
(knull? knil)
= #t
then
(fold kons knil (unfoldright knull? kar kdr x))
= x
and
(unfoldright knull? kar kdr (fold kons knil x))
= x.
This combinator presumably has some pretentious mathematical name; interested readers are invited to communicate it to the author.
map
proc clist_{1} clist_{2} ... > list
[R5RS+]
proc is a procedure taking as many arguments
as there are list arguments and returning a single value.
map
applies proc elementwise to the elements
of the lists and returns a list of the results,
in order.
The dynamic order in which proc
is applied to the elements of the lists is unspecified.
(map cadr '((a b) (d e) (g h))) => (b e h) (map (lambda (n) (expt n n)) '(1 2 3 4 5)) => (1 4 27 256 3125) (map + '(1 2 3) '(4 5 6)) => (5 7 9) (let ((count 0)) (map (lambda (ignored) (set! count (+ count 1)) count) '(a b))) => (1 2) or (2 1)
This procedure is extended from its R5RS specification to allow the arguments to be of unequal length; it terminates when the shortest list runs out.
At least one of the argument lists must be finite:
(map + '(3 1 4 1) (circularlist 1 0)) => (4 1 5 1)
foreach
proc clist_{1} clist_{2} ... > unspecified
[R5RS+]
The arguments to foreach
are like the arguments to
map
, but
foreach
calls proc for its side effects rather
than for its values.
Unlike map
, foreach
is guaranteed to call
proc on the elements of the lists in order from the first
element(s) to the last,
and the value returned by foreach
is unspecified.
(let ((v (makevector 5))) (foreach (lambda (i) (vectorset! v i (* i i))) '(0 1 2 3 4)) v) => #(0 1 4 9 16)
This procedure is extended from its R5RS specification to allow the arguments to be of unequal length; it terminates when the shortest list runs out.
At least one of the argument lists must be finite.
appendmap
f clist_{1} clist_{2} ... > value
appendmap!
f clist_{1} clist_{2} ... > value
Equivalent to
(apply append (map f clist_{1} clist_{2} ...))
and
(apply append! (map f clist_{1} clist_{2} ...))
Map f over the elements of the lists, just as in the map
function.
However, the results of the applications are appended together to
make the final result. appendmap
uses append
to append the results
together; appendmap!
uses append!
.
The dynamic order in which the various applications of f are made is not specified.
Example:
(appendmap! (lambda (x) (list x ( x))) '(1 3 8)) => (1 1 3 3 8 8)
At least one of the list arguments must be finite.
map!
f list_{1} clist_{2} ... > list
Linearupdate variant of map
 map!
is allowed, but not required, to
alter the cons cells of list_{1} to construct the result list.
The dynamic order in which the various applications of f are made is not specified.
In the nary case, clist_{2}, clist_{3}, ... must have at least as many elements as list_{1}.
mapinorder
f clist_{1} clist_{2} ... > list
A variant of the map
procedure that guarantees to apply f across
the elements of the list_{i} arguments in a lefttoright order. This
is useful for mapping procedures that both have side effects and
return useful values.
At least one of the list arguments must be finite.
pairforeach
f clist_{1} clist_{2} ... > unspecific
Like foreach
, but f is applied to successive sublists of the argument
lists. That is, f is applied to the cons cells of the lists, rather
than the lists' elements. These applications occur in lefttoright
order.
The f procedure may reliably apply setcdr!
to the pairs it is given
without altering the sequence of execution.
(pairforeach (lambda (pair) (display pair) (newline)) '(a b c)) ==> (a b c) (b c) (c)
At least one of the list arguments must be finite.
filtermap
f clist_{1} clist_{2} ... > list
Like map
, but only true values are saved.
(filtermap (lambda (x) (and (number? x) (* x x))) '(a 1 b 3 c 7)) => (1 9 49)
The dynamic order in which the various applications of f are made is not specified.
At least one of the list arguments must be finite.
filter
pred list > list
Return all the elements of list that satisfy predicate pred. The list is not disordered  elements that appear in the result list occur in the same order as they occur in the argument list. The returned list may share a common tail with the argument list. The dynamic order in which the various applications of pred are made is not specified.
(filter even? '(0 7 8 8 43 4)) => (0 8 8 4)
partition
pred list > [list list]
Partitions the elements of list with predicate pred, and returns two values: the list of inelements and the list of outelements. The list is not disordered  elements occur in the result lists in the same order as they occur in the argument list. The dynamic order in which the various applications of pred are made is not specified. One of the returned lists may share a common tail with the argument list.
(partition symbol? '(one 2 3 four five 6)) => (one four five) (2 3 6)
remove
pred list > list
Returns list without the elements that satisfy predicate pred:
(lambda (pred list) (filter (lambda (x) (not (pred x))) list))
The list is not disordered  elements that appear in the result list occur in the same order as they occur in the argument list. The returned list may share a common tail with the argument list. The dynamic order in which the various applications of pred are made is not specified.
(remove even? '(0 7 8 8 43 4)) => (7 43)
filter!
pred list > list
partition!
pred list > [list list]
remove!
pred list > list
Linearupdate variants of filter
, partition
and remove
.
These procedures are allowed, but not required, to alter the cons cells
in the argument list to construct the result lists.
The following procedures all search lists for a leftmost element satisfying some criteria. This means they do not always examine the entire list; thus, there is no efficient way for them to reliably detect and signal an error when passed a dotted or circular list. Here are the general rules describing how these procedures work when applied to different kinds of lists:
The standard, canonical behavior happens in this case.
It is an error to pass these procedures a dotted list that does not contain an element satisfying the search criteria. That is, it is an error if the procedure has to search all the way to the end of the dotted list. However, this SRFI does not specify anything at all about the behavior of these procedures when passed a dotted list containing an element satisfying the search criteria. It may finish successfully, signal an error, or perform some third action. Different implementations may provide different functionality in this case; code which is compliant with this SRFI may not rely on any particular behavior. Future SRFI's may refine SRFI1 to define specific behavior in this case.
In brief, SRFI1 compliant code may not pass a dotted list argument to these procedures.
It is an error to pass these procedures a circular list that does not contain an element satisfying the search criteria. Note that the procedure is not required to detect this case; it may simply diverge. It is, however, acceptable to search a circular list if the search is successful  that is, if the list contains an element satisfying the search criteria.
Here are some examples, using the find
and any
procedures as canonical
representatives:
;; Proper list  success (find even? '(1 2 3)) => 2 (any even? '(1 2 3)) => #t ;; proper list  failure (find even? '(1 7 3)) => #f (any even? '(1 7 3)) => #f ;; Failure is error on a dotted list. (find even? '(1 3 . x)) => error (any even? '(1 3 . x)) => error ;; The dotted list contains an element satisfying the search. ;; This case is not specified  it could be success, an error, ;; or some third possibility. (find even? '(1 2 . x)) => error/undefined (any even? '(1 2 . x)) => error/undefined ; success, error or other. ;; circular list  success (find even? (circularlist 1 6 3)) => 6 (any even? (circularlist 1 6 3)) => #t ;; circular list  failure is error. Procedure may diverge. (find even? (circularlist 1 3)) => error (any even? (circularlist 1 3)) => error
find
pred clist > value
Return the first element of clist that satisfies predicate pred; false if no element does.
(find even? '(3 1 4 1 5 9)) => 4
Note that find
has an ambiguity in its lookup semantics  if find
returns #f
, you cannot tell (in general) if it found a #f
element
that satisfied pred, or if it did not find any element at all. In
many situations, this ambiguity cannot arise  either the list being
searched is known not to contain any #f
elements, or the list is
guaranteed to have an element satisfying pred. However, in cases
where this ambiguity can arise, you should use findtail
instead of
find
 findtail
has no such ambiguity:
(cond ((findtail pred lis) => (lambda (pair) ...)) ; Handle (CAR PAIR) (else ...)) ; Search failed.
findtail
pred clist > pair or false
Return the first pair of clist whose car satisfies pred. If no pair does, return false.
findtail
can be viewed as a generalpredicate variant of the member
function.
Examples:
(findtail even? '(3 1 37 8 5 0 0)) => (8 5 0 0) (findtail even? '(3 1 37 5)) => #f ;; MEMBER X LIS: (findtail (lambda (elt) (equal? x elt)) lis)
In the circularlist case, this procedure "rotates" the list.
Findtail
is essentially dropwhile
,
where the sense of the predicate is inverted:
Findtail
searches until it finds an element satisfying
the predicate; dropwhile
searches until it finds an
element that doesn't satisfy the predicate.
takewhile
pred clist > list
takewhile!
pred clist > list
Returns the longest initial prefix of clist whose elements all satisfy the predicate pred.
Takewhile!
is the linearupdate variant. It is allowed, but not
required, to alter the argument list to produce the result.
(takewhile even? '(2 18 3 10 22 9)) => (2 18)
dropwhile
pred clist > list
Drops the longest initial prefix of clist whose elements all satisfy the predicate pred, and returns the rest of the list.
(dropwhile even? '(2 18 3 10 22 9)) => (3 10 22 9)
The circularlist case may be viewed as "rotating" the list.
span
pred clist > [list clist]
span!
pred list > [list list]
break
pred clist > [list clist]
break!
pred list > [list list]
Span
splits the list into the longest initial prefix whose
elements all satisfy pred, and the remaining tail.
Break
inverts the sense of the predicate:
the tail commences with the first element of the input list
that satisfies the predicate.
In other words:
span
finds the intial span of elements
satisfying pred,
and break
breaks the list at the first element satisfying
pred.
Span
is equivalent to
(values (takewhile pred clist) (dropwhile pred clist))
Span!
and break!
are the linearupdate variants.
They are allowed, but not required,
to alter the argument list to produce the result.
(span even? '(2 18 3 10 22 9)) => (2 18) (3 10 22 9) (break even? '(3 1 4 1 5 9)) => (3 1) (4 1 5 9)
any
pred clist_{1} clist_{2} ... > value
Applies the predicate across the lists, returning true if the predicate returns true on any application.
If there are n list arguments clist_{1} ... clist_{n}, then pred must be a
procedure taking n arguments
and returning a single value, interpreted as a boolean (that is,
#f
means false, and any other value means true).
any
applies pred to the first elements of the clist_{i} parameters.
If this application returns a true value, any
immediately returns
that value. Otherwise, it iterates, applying pred to the second
elements of the clist_{i} parameters, then the third, and so forth.
The iteration stops when a true value is produced or one of the lists runs
out of values; in
the latter case, any
returns #f
.
The application of pred to the last element of the
lists is a tail call.
Note the difference between find
and any
 find
returns the element
that satisfied the predicate; any
returns the true value that the
predicate produced.
Like every
, any
's name does not end with a question mark  this is to
indicate that it does not return a simple boolean (#t
or #f
), but a
general value.
(any integer? '(a 3 b 2.7)) => #t (any integer? '(a 3.1 b 2.7)) => #f (any < '(3 1 4 1 5) '(2 7 1 8 2)) => #t
every
pred clist_{1} clist_{2} ... > value
Applies the predicate across the lists, returning true if the predicate returns true on every application.
If there are n list arguments clist_{1} ... clist_{n}, then pred must be a
procedure taking n arguments
and returning a single value, interpreted as a boolean (that is,
#f
means false, and any other value means true).
every
applies pred to the first elements of the clist_{i} parameters.
If this application returns false, every
immediately returns false.
Otherwise, it iterates, applying pred to the second elements of the
clist_{i} parameters, then the third, and so forth. The iteration stops
when a false value is produced or one of the lists runs out of values.
In the latter case, every
returns
the true value produced by its final application of pred.
The application of pred to the last element of the lists
is a tail call.
If one of the clist_{i} has no elements, every
simply returns #t
.
Like any
, every
's name does not end with a question mark  this is to
indicate that it does not return a simple boolean (#t
or #f
), but a
general value.
listindex
pred clist_{1} clist_{2} ... > integer or false
Return the index of the leftmost element that satisfies pred.
If there are n list arguments clist_{1} ... clist_{n}, then pred must be a
function taking n arguments
and returning a single value, interpreted as a boolean (that is,
#f
means false, and any other value means true).
listindex
applies pred to the first elements of the clist_{i} parameters.
If this application returns true, listindex
immediately returns zero.
Otherwise, it iterates, applying pred to the second elements of the
clist_{i} parameters, then the third, and so forth. When it finds a tuple of
list elements that cause pred to return true, it stops and returns the
zerobased index of that position in the lists.
The iteration stops when one of the lists runs out of values; in this
case, listindex
returns #f
.
(listindex even? '(3 1 4 1 5 9)) => 2 (listindex < '(3 1 4 1 5 9 2 5 6) '(2 7 1 8 2)) => 1 (listindex = '(3 1 4 1 5 9 2 5 6) '(2 7 1 8 2)) => #f
member
x list [=] > list
memq
x list > list
memv
x list > list
[R5RS+]
These procedures return the first sublist of list whose car is
x, where the sublists of list are the
nonempty lists returned by
(drop list i)
for i less than the length of list.
If x does
not occur in list, then #f
is returned.
memq
uses eq?
to compare x
with the elements of list,
while memv
uses eqv?
, and
member
uses equal?
.
(memq 'a '(a b c)) => (a b c) (memq 'b '(a b c)) => (b c) (memq 'a '(b c d)) => #f (memq (list 'a) '(b (a) c)) => #f (member (list 'a) '(b (a) c)) => ((a) c) (memq 101 '(100 101 102)) => *unspecified* (memv 101 '(100 101 102)) => (101 102)
member
is extended from its
R5RS
definition to allow the client to pass in
an optional equality procedure = used to compare keys.
The comparison procedure is used to compare the elements e_{i} of list to the key x in this way:
(= x e_{i}) ; list is (E1 ... En)
That is, the first argument is always x, and the second argument is
one of the list elements. Thus one can reliably find the first element
of list that is greater than five with
(member 5 list <)
Note that fully general list searching may be performed with
the findtail
and find
procedures, e.g.
(findtail even? list) ; Find the first elt with an even key.
delete
x list [=] > list
delete!
x list [=] > list
delete
uses the comparison procedure =, which defaults to equal?
, to find
all elements of list that are equal to x, and deletes them from list. The
dynamic order in which the various applications of = are made is not
specified.
The list is not disordered  elements that appear in the result list occur in the same order as they occur in the argument list. The result may share a common tail with the argument list.
Note that fully general element deletion can be performed with the remove
and remove!
procedures, e.g.:
;; Delete all the even elements from LIS: (remove even? lis)
The comparison procedure is used in this way:
(= x e_{i})
.
That is, x is always the first argument,
and a list element is always the
second argument. The comparison procedure will be used to compare each
element of list exactly once; the order in which it is applied to the
various e_{i} is not specified. Thus, one can reliably remove all the
numbers greater than five from a list with
(delete 5 list <)
delete!
is the linearupdate variant of delete
.
It is allowed, but not required, to alter the cons cells in
its argument list to construct the result.
deleteduplicates
list [=] > list
deleteduplicates!
list [=] > list
deleteduplicates
removes duplicate elements from the
list argument.
If there are multiple equal elements in the argument list, the result list
only contains the first or leftmost of these elements in the result.
The order of these surviving elements is the same as in the original
list  deleteduplicates
does not disorder the list (hence it is useful
for "cleaning up" association lists).
The = parameter is used to compare the elements of the list; it defaults
to equal?
. If x comes before y in list, then the comparison is performed
(= x y)
.
The comparison procedure will be used to compare each pair of elements in
list no more than once;
the order in which it is applied to the various pairs is not specified.
Implementations of deleteduplicates
are allowed to share common tails
between argument and result lists  for example, if the list argument
contains only unique elements, it may simply return exactly
this list.
Be aware that, in general, deleteduplicates
runs in time O(n^{2}) for nelement lists.
Uniquifying long lists can be accomplished in O(n lg n) time by sorting
the list to bring equal elements together, then using a lineartime
algorithm to remove equal elements. Alternatively, one can use algorithms
based on elementmarking, with lineartime results.
deleteduplicates!
is the linearupdate variant of deleteduplicates
; it
is allowed, but not required, to alter the cons cells in its argument
list to construct the result.
(deleteduplicates '(a b a c a b c z)) => (a b c z) ;; Clean up an alist: (deleteduplicates '((a . 3) (b . 7) (a . 9) (c . 1)) (lambda (x y) (eq? (car x) (car y)))) => ((a . 3) (b . 7) (c . 1))
An "association list" (or "alist") is a list of pairs. The car of each pair contains a key value, and the cdr contains the associated data value. They can be used to construct simple lookup tables in Scheme. Note that association lists are probably inappropriate for performancecritical use on large data; in these cases, hash tables or some other alternative should be employed.
assoc
key alist [=] > pair or #f
assq
key alist > pair or #f
assv
key alist > pair or #f
[R5RS+]
alist must be an association list  a list of pairs.
These procedures
find the first pair in alist whose car field is key,
and returns that pair.
If no pair in alist has key as its car,
then #f
is returned.
assq
uses eq?
to compare key
with the car fields of the pairs in alist,
while assv
uses eqv?
and assoc
uses equal?
.
(define e '((a 1) (b 2) (c 3))) (assq 'a e) => (a 1) (assq 'b e) => (b 2) (assq 'd e) => #f (assq (list 'a) '(((a)) ((b)) ((c)))) => #f (assoc (list 'a) '(((a)) ((b)) ((c)))) => ((a)) (assq 5 '((2 3) (5 7) (11 13))) => *unspecified* (assv 5 '((2 3) (5 7) (11 13))) => (5 7)
assoc
is extended from its
R5RS
definition to allow the client to pass in
an optional equality procedure = used to compare keys.
The comparison procedure is used to compare the elements e_{i} of list to the key parameter in this way:
(= key (car e_{i})) ; list is (E1 ... En)
That is, the first argument is always key,
and the second argument is one of the list elements.
Thus one can reliably find the first entry
of alist whose key is greater than five with
(assoc 5 alist <)
Note that fully general alist searching may be performed with
the findtail
and find
procedures, e.g.
;; Look up the first association in alist with an even key: (find (lambda (a) (even? (car a))) alist)
alistcons
key datum alist > alist
(lambda (key datum alist) (cons (cons key datum) alist))
Cons a new alist entry mapping key > datum onto alist.
alistcopy
alist > alist
Make a fresh copy of alist. This means copying each pair that forms an association as well as the spine of the list, i.e.
(lambda (a) (map (lambda (elt) (cons (car elt) (cdr elt))) a))
alistdelete
key alist [=] > alist
alistdelete!
key alist [=] > alist
alistdelete
deletes all associations from alist with the given key,
using keycomparison procedure =, which defaults to equal?
.
The dynamic order in which the various applications of = are made is not
specified.
Return values may share common tails with the alist argument. The alist is not disordered  elements that appear in the result alist occur in the same order as they occur in the argument alist.
The comparison procedure is used to compare the element keys k_{i} of alist's
entries to the key parameter in this way:
(= key k_{i})
.
Thus, one can reliably remove all entries of alist whose key is greater
than five with
(alistdelete 5 alist <)
alistdelete!
is the linearupdate variant of alistdelete
.
It is allowed, but not required,
to alter cons cells from the alist parameter to construct the result.
These procedures implement operations on sets represented as lists of elements.
They all take an = argument used to compare elements of lists.
This equality procedure is required to be consistent with eq?
.
That is, it must be the case that
(eq? x y)
=> (= x y)
.
Note that this implies, in turn, that two lists that are eq?
are
also setequal by any legal comparison procedure. This allows for
constanttime determination of set operations on eq?
lists.
Be aware that these procedures typically run in time O(n * m) for n and melement list arguments. Performancecritical applications operating upon large sets will probably wish to use other data structures and algorithms.
lset<=
= list_{1} ... > boolean
Returns true iff every list_{i} is a subset of list_{i+1}, using = for the elementequality procedure. List A is a subset of list B if every element in A is equal to some element of B. When performing an element comparison, the = procedure's first argument is an element of A; its second, an element of B.
(lset<= eq? '(a) '(a b a) '(a b c c)) => #t (lset<= eq?) => #t ; Trivial cases (lset<= eq? '(a)) => #t
lset=
= list_{1} list_{2} ... > boolean
Returns true iff every list_{i} is setequal to list_{i+1}, using = for the elementequality procedure. "Setequal" simply means that list_{i} is a subset of list_{i+1}, and list_{i+1} is a subset of list_{i}. The = procedure's first argument is an element of list_{i}; its second is an element of list_{i+1}.
(lset= eq? '(b e a) '(a e b) '(e e b a)) => #t (lset= eq?) => #t ; Trivial cases (lset= eq? '(a)) => #t
Note added on 20200602: The reference (sample) implementation had a bug that reversed the arguments to =. The implementation has been corrected to match the text above.
lsetadjoin
= list elt_{1} ... > list
Adds the elt_{i} elements not already in the list parameter to the result list. The result shares a common tail with the list parameter. The new elements are added to the front of the list, but no guarantees are made about their order. The = parameter is an equality procedure used to determine if an elt_{i} is already a member of list. Its first argument is an element of list; its second is one of the elt_{i}.
The list parameter is always a suffix of the result  even if the list parameter contains repeated elements, these are not reduced.
(lsetadjoin eq? '(a b c d c e) 'a 'e 'i 'o 'u) => (u o i a b c d c e)
lsetunion
= list_{1} ... > list
Returns the union of the lists, using = for the elementequality procedure.
The union of lists A and B is constructed as follows:
(= r b)
.
If all comparisons fail,
b is consed onto the front of the result.
However, there is no guarantee that = will be applied to every pair
of arguments from A and B.
In particular, if A is eq
? to B,
the operation may immediately terminate.
In the nary case, the twoargument listunion operation is simply folded across the argument lists.
(lsetunion eq? '(a b c d e) '(a e i o u)) => (u o i a b c d e) ;; Repeated elements in LIST1 are preserved. (lsetunion eq? '(a a c) '(x a x)) => (x a a c) ;; Trivial cases (lsetunion eq?) => () (lsetunion eq? '(a b c)) => (a b c)
lsetintersection
= list_{1} list_{2} ... > list
Returns the intersection of the lists, using = for the elementequality procedure.
The intersection of lists A and B
is comprised of every element of A that is =
to some element of B:
(= a b)
,
for a in A, and b in B.
Note this implies that an element which appears in B
and multiple times in list A
will also appear multiple times in the result.
The order in which elements appear in the result is the same as
they appear in list_{1} 
that is, lsetintersection
essentially filters
list_{1},
without disarranging element order.
The result may
share a common tail with list_{1}.
In the nary case, the twoargument listintersection operation is simply folded across the argument lists. However, the dynamic order in which the applications of = are made is not specified. The procedure may check an element of list_{1} for membership in every other list before proceeding to consider the next element of list_{1}, or it may completely intersect list_{1} and list_{2} before proceeding to list_{3}, or it may go about its work in some third order.
(lsetintersection eq? '(a b c d e) '(a e i o u)) => (a e) ;; Repeated elements in LIST1 are preserved. (lsetintersection eq? '(a x y a) '(x a x z)) => '(a x a) (lsetintersection eq? '(a b c)) => (a b c) ; Trivial case
lsetdifference
= list_{1} list_{2} ... > list
Returns the difference of the lists, using = for the elementequality procedure  all the elements of list_{1} that are not = to any element from one of the other list_{i} parameters.
The = procedure's first argument is
always an element of list_{1};
its second is an element of one of the other list_{i}.
Elements that are repeated multiple times in the
list_{1} parameter
will occur multiple times in the result.
The order in which elements appear in the result is the same as
they appear in list_{1} 
that is, lsetdifference
essentially
filters list_{1}, without disarranging element order.
The result may share a common tail with list_{1}.
The dynamic order in which the applications of = are made is not
specified.
The procedure may check an element of list_{1}
for membership in every other list before proceeding to consider the
next element of list_{1},
or it may completely compute the difference of
list_{1} and list_{2} before
proceeding to list_{3},
or it may go about its work in some third order.
(lsetdifference eq? '(a b c d e) '(a e i o u)) => (b c d) (lsetdifference eq? '(a b c)) => (a b c) ; Trivial case
lsetxor
= list_{1} ... > list
Returns the exclusiveor of the sets, using = for the elementequality procedure. If there are exactly two lists, this is all the elements that appear in exactly one of the two lists. The operation is associative, and thus extends to the nary case  the elements that appear in an odd number of the lists. The result may share a common tail with any of the list_{i} parameters.
More precisely, for two lists A and B, A xor B is a list of
(= a b)
, and
(= b a)
.
However, an implementation is allowed to assume that = is symmetric  that is, that
(= a b)
=>
(= b a)
.
This means, for example, that if a comparison
(= a b)
produces
true for some a in A
and b in B,
both a and b may be removed from
inclusion in the result.
In the nary case, the binaryxor operation is simply folded across the lists.
(lsetxor eq? '(a b c d e) '(a e i o u)) => (d c b i o u) ;; Trivial cases. (lsetxor eq?) => () (lsetxor eq? '(a b c d e)) => (a b c d e)
lsetdiff+intersection
= list_{1} list_{2} ... > [list list]
Returns two values  the difference and the intersection of the lists. Is equivalent to
(values (lsetdifference = list_{1} list_{2} ...) (lsetintersection = list_{1} (lsetunion = list_{2} ...)))
but can be implemented more efficiently.
The = procedure's first argument is an element of list_{1}; its second is an element of one of the other list_{i}.
Either of the answer lists may share a common tail with list_{1}. This operation essentially partitions list_{1}.
lsetunion!
= list_{1} ... > list
lsetintersection!
= list_{1} list_{2} ... > list
lsetdifference!
= list_{1} list_{2} ... > list
lsetxor!
= list_{1} ... > list
lsetdiff+intersection!
= list_{1} list_{2} ... > [list list]
These are linearupdate variants. They are allowed, but not required,
to use the cons cells in their first list parameter to construct their
answer. lsetunion!
is permitted to recycle cons cells from any
of its list arguments.
These two procedures are the primitive, R5RS sideeffect operations on pairs.
setcar!
pair object > unspecified
setcdr!
pair object > unspecified
[R5RS] These procedures store object in the car and cdr field of pair, respectively. The value returned is unspecified.
(define (f) (list 'notaconstantlist)) (define (g) '(constantlist)) (setcar! (f) 3) => *unspecified* (setcar! (g) 3) => *error*
The design of this library benefited greatly from the feedback provided during the SRFI discussion phase. Among those contributing thoughtful commentary and suggestions, both on the mailing list and by private discussion, were Mike Ashley, Darius Bacon, Alan Bawden, Phil Bewig, Jim Blandy, Dan Bornstein, Per Bothner, Anthony Carrico, Doug Currie, Kent Dybvig, Sergei Egorov, Doug Evans, Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Dan Friedman, Lars Thomas Hansen, Brian Harvey, Erik Hilsdale, Wolfgang Hukriede, Richard Kelsey, Donovan Kolbly, Shriram Krishnamurthi, Dave Mason, Jussi Piitulainen, David Pokorny, Duncan Smith, Mike Sperber, Maciej Stachowiak, Harvey J. Stein, John David Stone, and Joerg F. Wittenberger. I am grateful to them for their assistance.
I am also grateful the authors, implementors and documentors of all the systems mentioned in the rationale. Aubrey Jaffer and Kent Pitman should be noted for their work in producing Webaccessible versions of the R5RS and Common Lisp spec, which was a tremendous aid.
This is not to imply that these individuals necessarily endorse the final results, of course.
Common Lisp: the Language
Guy L. Steele Jr. (editor).
Digital Press, Maynard, Mass., second edition 1990.
(Wikipedia entry)
The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially the ANSI spec for Common Lisp: http://www.lispworks.com/documentation/HyperSpec/Front/index.htm
Certain portions of this document  the specific, marked segments of text describing the R5RS procedures  were adapted with permission from the R5RS report.
All other text is copyright (C) Olin Shivers (1998, 1999). All Rights Reserved.
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.