SRFI-116: Immutable Lists

Scheme currently does not provide immutable pairs corresponding to its existing mutable pairs, although most uses of pairs do not exploit their mutability. The Racket system takes the radical approach of making Scheme's pairs immutable, and providing a minimal library of mutable pairs with procedures named mpair?, mcons, mcar, mcdr, set-mcar!, set-mcdr!. This SRFI takes the opposite approach of leaving Scheme's pairs unchanged and providing a full set of routines for creating and dealing with immutable pairs. The sample implementation is portable (to systems with SRFI 9) and efficient.

  1. SRFI-116: Immutable Lists
  2. Installation
  3. SRFI Description
  4. Immutable Lists
    1. Constructors
    2. Predicates
    3. Selectors
    4. Miscellaneous
    5. Fold, unfold & map
    6. Filtering & partitioning
    7. Searching
    8. Deleting
    9. Immutable association lists
    10. Replacement
    11. Conversion
    12. Procedure application
    13. Comparators
  5. Repository
  6. Version History
  7. License

Installation

$ chicken-install srfi-116

or

$ chicken-install srfi-116 -test

if you want to run the tests for the egg in addition.

SRFI Description

For a full description of this SRFI, see the full SRFI document. This documentation covers the API only.

Immutable Lists

Constructors

[procedure] (ipair a d)

The primitive constructor. Returns a newly allocated ipair whose icar is a and whose icdr is d. The ipair is guaranteed to be different (in the sense of eqv?) from every existing object.

(ipair 'a '())            ;=> (a)
(ipair (iq a) (iq b c d)) ;=> ((a) b c d)
(ipair "a" (iq b c))      ;=> ("a" b c)
(ipair 'a 3)              ;=> (a . 3)
(ipair (iq a b) 'c)       ;=> ((a b ) . c)
[procedure] (ilist obj ...)

Returns a newly allocated ilist of its arguments.

(ilist 'a (+ 3 4) 'c) ;=>  (a 7 c)
(ilist)               ;=>  ()
[procedure] (xipair d a)
(lambda (d a) (ipair a d))

Of utility only as a value to be conveniently passed to higher-order procedures.

(xipair (iq b c) 'a) ;=> (a b c)

The name stands for "eXchanged Immutable PAIR."

[procedure] (ipair* elt1 elt2 ...)

Like ilist, but the last argument provides the tail of the constructed ilist, returning (ipair elt1 (ipair elt2 (ipair ... eltn)))

(ipair* 1 2 3 4) ;=> (1 2 3 . 4)
(ipair* 1)       ;=> 1
[procedure] (make-ilist n [fill])

Returns an n-element ilist, whose elements are all the value fill. If the fill argument is not given, the elements of the ilist may be arbitrary values.

(make-ilist 4 'c) ;=> (c c c c)
[procedure] (ilist-tabulate n init-proc)

Returns an n-element ilist. Element i of the ilist, where 0 <= i < n, is produced by (init-proc i). No guarantee is made about the dynamic order in which init-proc is applied to these indices.

(ilist-tabulate 4 values) ;=> (0 1 2 3)
[procedure] (ilist-copy dilist)

Copies the spine of the argument, including the ilist tail.

[procedure] (iiota count [start [step]])

Returns an ilist containing the elements:

(start start+step ... start+(count-1)*step)

The start and step parameters default to 0 and 1, respectively. This procedure takes its name from the APL primitive.

(iiota 5)        ;=> (0 1 2 3 4)
(iiota 5 0 -0.1) ;=> (0 -0.1 -0.2 -0.3 -0.4)

Predicates

[procedure] (proper-ilist? x)
[procedure] (ilist? x)

These identifiers are bound either to the same procedure, or to procedures of equivalent behavior. In either case, true is returned iff x is a proper ilist — a ()-terminated ilist.

More carefully: The empty list is a proper ilist. An ipair whose icdr is a proper ilist is also a proper ilist. Everything else is a dotted ilist. This includes non-ipair, non-() values (e.g. symbols, numbers, mutable pairs), which are considered to be dotted ilists of length 0.

[procedure] (dotted-ilist? x)

True if x is a finite, non-nil-terminated ilist. That is, there exists an n >= 0 such that (icdrn x) is neither an ipair nor (). This includes non-ipair, non-() values (e.g. symbols, numbers), which are considered to be dotted ilists of length 0.

(dotted-ilist? x) = (not (proper-ilist? x))
[procedure] (ipair? obj)

Returns #t if object is an ipair; otherwise #f

(ipair? (ipair 'a 'b)) ;=>  #t
(ipair? (iq a b c))    ;=>  #t
(ipair? (cons 1 2))    ;=>  #f
(ipair? '())           ;=>  #f
(ipair? '#(a b))       ;=>  #f
(ipair? 7)             ;=>  #f
(ipair? 'a)            ;=>  #f
[procedure] (null-ilist? ilist)

ilist is a proper ilist. 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 ilist. This procedure is recommended as the termination condition for ilist-processing procedures that are not defined on dotted ilists.

[procedure] (not-ipair? x)
(lambda (x) (not (ipair? x)))

Provided as a procedure as it can be useful as the termination condition for ilist-processing procedures that wish to handle all ilists, both proper and dotted.

[procedure] (ilist= elt= ilist1 ...)

Determines ilist equality, given an element-equality procedure. Proper ilist A equals proper ilist B if they are of the same length, and their corresponding elements are equal, as determined by elt=. If the element-comparison procedure's first argument is from ilisti, then its second argument is from ilisti+1, i.e. it is always called as (elt= a b) for a an element of ilist A, and b an element of ilist B.

In the n-ary case, every ilisti is compared to ilisti+1 (as opposed, for example, to comparing ilist1 to ilisti, for i>1). If there are no ilist arguments at all, ilist= simply returns true.

It is an error to apply ilist= to anything except proper ilists. It cannot reasonably be extended to dotted ilists, as it provides no way to specify an equality procedure for comparing the ilist terminators.

Note that the dynamic order in which the elt= procedure is applied to pairs of elements is not specified. For example, if ilist= is applied to three ilists, 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 ilists which are eq? are always ilist=, as well; implementations may exploit this fact to "short-cut" the element-by-element comparisons.

(ilist= eq?)        ;=> #t ;Trivial cases
(ilist= eq? (iq a)) ;=> #t

Selectors

[procedure] (icar ipair)
[procedure] (icdr ipair)

These procedures return the contents of the icar and icdr field of their argument, respectively. Note that it is an error to apply them to the empty ilist.

(icar (iq a b c))       ;=>  a
(icar (iq (a) b c d))   ;=>  (a)
(icar (ipair 1 2))      ;=>  1
(icar '())              ;=>  *error*

(icdr (iq a b c))       ;=>  (b c)
(icdr (iq (a) b c d))   ;=>  (b c d)
(icdr (ipair 1 2))      ;=>  2
(icdr '())              ;=>  *error*
[procedure] (icaar ipair)
[procedure] (icadr ipair)
[procedure] (icdddar ipair)
[procedure] (icddddr ipair)

These procedures are compositions of icar and icdr, where for example icaddr could be defined by:

(define icaddr (lambda (x) (icar (icdr (icdr x))))).

Arbitrary compositions, up to four deep, are provided. There are twenty-eight of these procedures in all.

[procedure] (ilist-ref ilist i)

Returns the ith element of ilist. (This is the same as the icar of (idrop ilist i).) It is an error if i >= n, where n is the length of ilist.

(ilist-ref (iq a b c d) 2) ;=> c
[procedure] (ifirst ipair)
[procedure] (isecond ipair)
[procedure] (ithird ipair)
[procedure] (ifourth ipair)
[procedure] (ififth ipair)
[procedure] (isixth ipair)
[procedure] (iseventh ipair)
[procedure] (ieigth ipair)
[procedure] (ininth ipair)
[procedure] (itenth ipair)

Synonyms for car, cadr, caddr, ...

(ithird '(a b c d e)) ;=> c

<procedure>(icar+icdr ipair)<procedure>

The fundamental ipair deconstructor:

(lambda (p) (values (icar p) (icdr p)))

This can, of course, be implemented more efficiently by a compiler.

[procedure] (itake x i)
[procedure] (idrop x i)
[procedure] (ilist-tail x i)

itake returns the first i elements of ilist x. idrop returns all but the first i elements of ilist x. ilist-tail is either the same procedure as idrop or else a procedure with the same behavior.

(itake (iq a b c d e)  2) ;=> (a b)
(idrop (iq a b c d e)  2) ;=> (c d e)

x may be any value — a proper or dotted ilist:

(itake (ipair 1 (ipair 2 (ipair 3 'd)))    ;=> (1 2)
(idrop (ipair 1 (ipair 2 (ipair 3 'd))) 2) ;=> (3 . d)
(itake (ipair 1 (ipair 2 (ipair 3 'd))) 3) ;=> (1 2 3)
(idrop (ipair 1 (ipair 2 (ipair 3 'd))) 3) ;=> d

For a legal i, itake and idrop partition the ilist in a manner which can be inverted with iappend:

(iappend (itake x i) (idrop x i)) = x

idrop is exactly equivalent to performing i icdr operations on x; the returned value shares a common tail with x.

[procedure] (itake-right dilist i)
[procedure] (idrop-right dilist i)

itake-right returns the last i elements of dilist. idrop-right returns all but the last i elements of dilist.

(itake-right (iq a b c d e) 2) ;=> (d e)
(idrop-right (iq a b c d e) 2) ;=> (a b c)

The returned ilist may share a common tail with the argument ilist.

dilist may be any ilist, either proper or dotted:

(itake-right (iq ipair 1 (ipair 2 (ipair 3 'd))) 2) ;=> (2 3 . d)
(idrop-right (ipair 1 (ipair 2 (ipair 3 'd))) 2)    ;=> (1)
(itake-right (ipair 1 (ipair 2 (ipair 3 'd))) 0)    ;=> d
(idrop-right (ipair 1 (ipair 2 (ipair 3 'd))) 0)    ;=> (1 2 3)

For a legal i, itake-right and idrop-right partition the ilist in a manner which can be inverted with iappend:

(iappend (itake dilist i) (idrop dilist i)) ;=> dilist

itake-right's return value is guaranteed to share a common tail with dilist.

[procedure] (isplit-at x i)

isplit-at splits the ilist x at index i, returning an ilist of the first i elements, and the remaining tail. It is equivalent to

(values (itake x i) (idrop x i))
[procedure] (ilast ipair)
[procedure] (last-ipair ipair)

Returns the last element of the non-empty, possibly dotted, ilist ipair. last-ipair returns the last ipair in the non-empty ilist pair.

(ilast (iq a b c))      ;=> c
(last-ipair (iq a b c)) ;=> (c)

Miscellaneous

[procedure] (ilength ilist)

Returns the length of its argument. It is an error to pass a value to ilength which is not a proper ilist (()-terminated).

The length of a proper ilist is a non-negative integer n such that icdr applied n times to the ilist produces the empty list.

[procedure] (iappend ilist1 ...)

Returns an ilist consisting of the elements of ilist1 followed by the elements of the other ilist parameters.

(iappend (iq x) (iq y))        ;=>  (x y)
(iappend (iq a) (iq b c d))    ;=>  (a b c d)
(iappend (iq a (b)) (iq (c)))  ;=>  (a (b) (c))

The resulting ilist is always newly allocated, except that it shares structure with the final ilisti argument. This last argument may be any value at all; an improper ilist results if it is not a proper ilist. All other arguments must be proper ilists.

(iappend (iq a b) (ipair 'c 'd))  ;=>  (a b c . d)
(iappend '() 'a)                  ;=>  a
(iappend (iq x y))                ;=>  (x y)
(iappend)                         ;=>  ()
[procedure] (iconcatenate ilist-of-ilists)

Appends the elements of its argument together. That is, iconcatenate returns

(iapply iappend ilist-of-ilists)

or, equivalently,

(ireduce-right iappend '() ilist-of-ilists)

Note that some Scheme implementations do not support passing more than a certain number (e.g., 64) of arguments to an n-ary procedure. In these implementations, the (iapply iappend ...) idiom would fail when applied to long lists, but iconcatenate would continue to function properly.

As with iappend, the last element of the input list may be any value at all.

[procedure] (ireverse ilist)

Returns a newly allocated ilist consisting of the elements of ilist in reverse order.

(ireverse (iq a b c))             ;=>  (c b a)
(ireverse (iq a (b c) d (e (f)))) ;=>  ((e (f)) d (b c) a)
[procedure] (iappend-reverse rev-head tail)

iappend-reverse returns (iappend (ireverse rev-head) tail). It is provided because it is a common operation — a common list-processing style calls for this exact operation to transfer values accumulated in reverse order onto the front of another ilist, 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 iappend-reverse 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.)

[procedure] (izip ilist1 ilist2 ...)
(lambda ilists (iapply imap ilist ilists))

If izip is passed n ilists, it returns an ilist as long as the shortest of these ilists, each element of which is an n-element ilist comprised of the corresponding elements from the parameter ilists.

(izip (iq one two three)
      (iq 1 2 3)
      (iq odd even odd even odd even odd even))
 ;=> ((one 1 odd) (two 2 even) (three 3 odd))

(izip (iq 1 2 3)) ;=> ((1) (2) (3))
[procedure] (iunzip1 ilist)
[procedure] (iunzip2 ilist)
[procedure] (iunzip3 ilist)
[procedure] (iunzip4 ilist)
[procedure] (iunzip5 ilist)

iunzip1 takes an ilist of ilists, where every ilist must contain at least one element, and returns an ilist containing the initial element of each such ilist. That is, it returns (imap icar ilists).

unzip2 takes an ilist of ilists, where every ilist must contain at least two elements, and returns two values: an ilist of the first elements, and an ilist of the second elements.

iunzip3 does the same for the first three elements of the ilists, and so forth.

(iunzip2 (iq (1 one) (2 two) (3 three)))
 ;=> (1 2 3)
 ;=> (one two three)
[procedure] (icount pred ilist1 ilist2 ...)

pred is a procedure taking as many arguments as there are ilists and returning a single value. It is applied element-wise to the elements of the ilists, 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 ilist elements in a left-to-right order. The counting stops when the shortest ilist expires.

(count even? (iq 3 1 4 1 5 9 2 5 6))            ;=> 3
(count < (iq 1 2 4 8) (iq 2 4 6 8 10 12 14 16)) ;=> 3

Fold, unfold & map

[procedure] (ifold konx knil ilist1 ilist2 ...)

The fundamental ilist iterator.

First, consider the single ilist-parameter case. If ilist1 is (e1 e2 ... en), then this procedure returns

(kons en ... (kons e2 (kons e1 knil)) ... )

That is, it obeys the (tail) recursion

(ifold kons knil lis) ;=> (ifold kons (kons (icar lis) knil) (icdr lis))
(ifold kons knil '()) ;=> knil

Examples:

(ifold + 0 lis)             ; Add up the elements of LIS.
(ifold ipair '() lis)       ; Reverse LIS.
(ifold ipair tail rev-head) ; See APPEND-REVERSE.

;; How many symbols in LIS?
(ifold (lambda (x count) (if (symbol? x) (+ count 1) count))
      0
      lis)

;; Length of the longest string in LIS:
(ifold (lambda (s max-len) (max max-len (string-length s)))
      0
      lis)

If n ilist arguments are provided, then the kons function must take n+1 parameters: one element from each ilist, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest ilist runs out of values:

(ifold ipair* '() (iq a b c) (iq 1 2 3 4 5)) ;=> (c 3 b 2 a 1)
[procedure] (ifold-right kons knil ilist1 ilist2 ...)

The fundamental ilist recursion operator.

First, consider the single ilist-parameter case. If ilist1 is (e1 e2 ... en), then this procedure returns

(kons e1 (kons e2 ... (kons en knil)))

That is, it obeys the recursion

(ifold-right kons knil lis) ;=> (kons (icar lis) (ifold-right kons knil (icdr lis)))
(ifold-right kons knil '()) ;=> knil

Examples:

(ifold-right ipair '() lis)     ; Copy LIS.

;; Filter the even numbers out of LIS.
(ifold-right (lambda (x l) (if (even? x) (ipair x l) l)) '() lis))

If n ilist arguments are provided, then the kons procedure must take n+1 parameters: one element from each ilist, and the "seed" or fold state, which is initially knil. The fold operation terminates when the shortest ilist runs out of values:

(ifold-right ipair* '() (iq a b c) (iq 1 2 3 4 5)) ;=> (a 1 b 2 c 3)
[procedure] (ipair-fold kons knil ilist1 ilist2 ...)

Analogous to fold, but kons is applied to successive sub-ilists of the ilists, rather than successive elements — that is, kons is applied to the ipairs making up the lists, giving this (tail) recursion:

(ipair-fold kons knil lis) ;=> (let ((tail (icdr lis)))
                           ;     (ipair-fold kons (kons lis knil) tail))
(ipair-fold kons knil '()) ;=> knil

Example:

(ipair-fold ipair '() (iq a b c)) ;=> ((c) (b c) (a b c))
[procedure] (ipair-fold-right kons knil ilist1 ilist2 ...)

Holds the same relationship with {{ifold-right} that {ipair-fold} holds with {ifold}. Obeys the recursion

(ipair-fold-right kons knil lis) ;=> (kons lis (ipair-fold-right kons knil (icdr lis)))
(ipair-fold-right kons knil '()) ;=> knil

Example:

(ipair-fold-right ipair '() (iq a b c)) ;=> ((a b c) (b c) (c))
[procedure] (ireduce f ridentity ilist)

ireduce is a variant of ifold.

ridentity should be a "right identity" of the procedure f — that is, for any value x acceptable to f,

(f x ridentity) ;=> x

ireduce has the following definition: If ilist is (), return ridentity; Otherwise, return (ifold f (icar ilist) (icdr ilist)). In other words, we compute (ifold f ridentity ilist).

Note that ridentity is used only in the empty-list case. You typically use ireduce when applying f is expensive and you'd like to avoid the extra application incurred when ifold applies f to the head of ilist 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, ifold is useful in many contexts where ireduce is not (consider the examples given in the ifold definition -- only one of the five folds uses a function with a right identity. The other four may not be performed with ireduce).

;; take the max of an ilist of non-negative integers.
(ireduce max 0 nums) ; i.e., (iapply max 0 nums)
[procedure] (ireduce-right f ridentity ilist)

ireduce-right is the fold-right variant of ireduce. It obeys the following definition:

(ireduce-right f ridentity '())            ;=> ridentity
(ireduce-right f ridentity (iq e1))        ;=> (f e1 ridentity) ;=> e1
(ireduce-right f ridentity (iq e1 e2 ...)) ;=> (f e1 (ireduce f ridentity (e2 ...)))

...in other words, we compute (ifold-right f ridentity ilist).

;; Append a bunch of ilists together.
;; I.e., (iapply iappend ilist-of-ilists)
(ireduce-right iappend '() ilist-of-ilists)
[procedure] (iunfold p f g seed [tail-gen])

iunfold is best described by its basic recursion:

(iunfold p f g seed) =
    (if (p seed) (tail-gen seed)
        (ipair (f seed)
              (iunfold p f g (g seed))))
p
Determines when to stop unfolding.
f
Maps each seed value to the corresponding ilist element.
g
Maps each seed value to next seed value.
seed
The "state" value for the unfold.
tail-gen
Creates the tail of the ilist; defaults to (lambda (x) '())

In other words, we use g to generate a sequence of seed values seed, (g seed), (g2 seed), (g3 seed), ... These seed values are mapped to ilist elements by f, producing the elements of the result ilist in a left-to-right order. P says when to stop (i.e. when it returns true the procedure ends).

iunfold is the fundamental recursive ilist constructor, just as ifold-right is the fundamental recursive ilist consumer. While iunfold may seem a bit abstract to novice functional programmers, it can be used in a number of ways:

;; Ilist of squares: 1^2 ... 10^2
(iunfold (lambda (x) (> x 10))
        (lambda (x) (* x x))
    (lambda (x) (+ x 1))
    1)

(iunfold null-ilist? icar icdr lis) ; Copy a proper ilist.

;; Read current input port into an ilist of values.
(iunfold eof-object? values (lambda (x) (read)) (read))

;; Copy a possibly non-proper ilist:
(iunfold not-ipair? icar icdr lis
              values)

;; Append HEAD onto TAIL:
(iunfold null-ilist? icar icdr head
              (lambda (x) tail))

Interested functional programmers may enjoy noting that ifold-right and iunfold 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

(ifold-right kons knil (iunfold knull? kar kdr x)) ;=> x ;AND
(iunfold knull? kar kdr (ifold-right kons knil x)) ;=> x

This combinator sometimes is called an "anamorphism;" when an explicit tail-gen procedure is supplied, it is called an "apomorphism."

[procedure] (iunfold-right p f g seed [tail])

iunfold-right constructs an ilist with the following loop:

(let lp ((seed seed) (lis tail))
  (if (p seed) lis
      (lp (g seed)
          (ipair (f seed) lis))))
p
Determines when to stop unfolding.
f
Maps each seed value to the corresponding ilist element.
g
Maps each seed value to next seed value.
seed
The "state" value for the unfold.
tail
ilist terminator; defaults to '().

In other words, we use g to generate a sequence of seed values seed, (g seed), (g2 seed), (g3 seed), ... These seed values are mapped to ilist elements by f, producing the elements of the result ilist in a right-to-left order. P says when to stop.

iunfold-right is the fundamental iterative ilist constructor, just as ifold is the fundamental iterative ilist consumer. While iunfold-right may seem a bit abstract to novice functional programmers, it can be used in a number of ways:

;; Ilist of squares: 1^2 ... 10^2
(iunfold-right zero?
              (lambda (x) (* x x))
              (lambda (x) (- x 1))
              10)

;; Reverse a proper ilist.
(iunfold-right null-ilist? icar icdr lis)

;; Read current input port into an ilist of values.
(iunfold-right eof-object? values (lambda (x) (read)) (read))

;; (iappend-reverse rev-head tail)
(iunfold-right null-ilist? icar icdr rev-head tail)
[procedure] (imap proc ilist1 ilist2 ...)

proc is a procedure taking as many arguments as there are ilist arguments and returning a single value. imap applies proc element-wise to the elements of the ilists and returns an ilist of the results, in order. The dynamic order in which proc is applied to the elements of the ilists is unspecified.

(imap icadr (iq (a b) (d e) (g h))) ;=>  (b e h)

(imap (lambda (n) (expt n n))
      (iq 1 2 3 4 5))
 ;=>  (1 4 27 256 3125)

(imap + (iq 1 2 3) (iq 4 5 6)) ;=>  (5 7 9)

(let ((count 0))
  (imap (lambda (ignored)
         (set! count (+ count 1))
         count)
       (iq a b))) ;=>  (1 2) or (2 1)
[procedure] (ifor-each proc ilist1 ilist2 ...)

The arguments to ifor-each are like the arguments to imap, but ifor-each calls proc for its side effects rather than for its values. Unlike imap, ifor-each is guaranteed to call proc on the elements of the ilists in order from the first element(s) to the last, and the value returned by ifor-each is unspecified.

(let ((v (make-vector 5)))
  (ifor-each (lambda (i)
              (vector-set! v i (* i i)))
            (iq 0 1 2 3 4))
  v)  ;=>  #(0 1 4 9 16)
[procedure] (iappend-map f ilist1 ilist2 ...)

Equivalent to (iapply iappend (imap f ilist1 ilist2 ...)) and (iapply iappend (imap f ilist1 ilist2 ...)). Map f over the elements of the ilists, just as in the imap function. However, the results of the applications are appended together (using iappend) to make the final result.

The dynamic order in which the various applications of f are made is not specified.

Example:

(iappend-map (lambda (x) (ilist x (- x))) (iq 1 3 8)) ;=> (1 -1 3 -3 8 -8)
[procedure] (imap-in-order f ilist1 ilist2 ...)

A variant of the imap procedure that guarantees to apply f across the elements of the ilisti arguments in a left-to-right order. This is useful for mapping procedures that both have side effects and return useful values.

[procedure] (ipair-for-each f ilist1 ilist2 ...)

Like ifor-each, but f is applied to successive sub-ilists of the argument ilists. That is, f is applied to the cells of the ilists, rather than the ilists' elements. These applications occur in left-to-right order.

(ipair-for-each (lambda (ipair) (display ipair) (newline)) (iq a b c))
 ;=> (a b c)
 ;=> (b c)
 ;=> (c)
[procedure] (ifilter-map f ilist1 ilist2 ...)

Like imap, but only true values are saved.

(ifilter-map (lambda (x) (and (number? x) (* x x))) (iq a 1 b 3 c 7))
 ;=> (1 9 49)

The dynamic order in which the various applications of f are made is not specified.

Filtering & partitioning

[procedure] (ifilter pred ilist)

Return all the elements of ilist that satisfy predicate pred. The ilist is not disordered — elements that appear in the result ilist occur in the same order as they occur in the argument ilist. The returned ilist may share a common tail with the argument ilist. The dynamic order in which the various applications of pred are made is not specified.

(ifilter even? (iq 0 7 8 8 43 -4)) ;=> (0 8 8 -4)
[procedure] (ipartition pred ilist)

Partitions the elements of ilist with predicate pred, and returns two values: the ilist of in-elements and the ilist of out-elements. The ilist is not disordered — elements occur in the result ilists in the same order as they occur in the argument ilist. The dynamic order in which the various applications of pred are made is not specified. One of the returned ilists may share a common tail with the argument ilist.

(ipartition symbol? (iq one 2 3 four five 6))
 ;=> (one four five)
 ;=> (2 3 6)
[procedure] (iremove pred ilist)

Returns ilist without the elements that satisfy predicate pred:

(lambda (pred ilist) (ifilter (lambda (x) (not (pred x))) ilist))

The ilist is not disordered — elements that appear in the result ilist occur in the same order as they occur in the argument ilist. The returned ilist may share a common tail with the argument ilist. The dynamic order in which the various applications of pred are made is not specified.

(iremove even? (iq 0 7 8 8 43 -4)) ;=> (7 43)

Searching

The following procedures all search ilists for a leftmost element satisfying some criteria. This means they do not always examine the entire ilist; thus, there is no efficient way for them to reliably detect and signal an error when passed a dotted ilist. Here are the general rules describing how these procedures work when applied to different kinds of ilists:

Proper ilists
The standard, canonical behavior happens in this case.
Dotted ilists
It is an error to pass these procedures a dotted ilist 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 ilist. However, this SRFI does not specify anything at all about the behavior of these procedures when passed a dotted ilist 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 SRFIs may refine this SRFI to define specific behavior in this case.

In brief, compliant code may not pass a dotted ilist argument to these procedures.

Here are some examples, using the ifind and iany procedures as canonical representatives:

;; Proper ilist — success
(ifind even? (iq 1 2 3))    ;=> 2
(iany  even? (iq 1 2 3))    ;=> #t

;; proper ilist — failure
(ifind even? (iq 1 7 3))    ;=> #f
(iany  even? (iq 1 7 3))    ;=> #f

;; Failure is error on a dotted ilist.
(ifind even? (ipair 1 (ipair 3 'x)))    ;=> error
(iany  even? (ipair 1 (ipair 3 'x)))    ;=> error

;; The dotted ilist contains an element satisfying the search.
;; This case is not specified — it could be success, an error,
;; or some third possibility.
(ifind even? (ipair 1 (ipair 2 'x)))    ;=> error/undefined
(iany  even? (ipair 1 (ipair 2 'x)))    ;=> error/undefined ; success, error or other.
[procedure] (ifind pred ilist)

Return the first element of ilist that satisfies predicate pred; false if no element does.

(ifind even? (iq 3 1 4 1 5 9)) ;=> 4

Note that ifind has an ambiguity in its lookup semantics — if ifind 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 ilist being searched is known not to contain any #f elements, or the ilist is guaranteed to have an element satisfying pred. However, in cases where this ambiguity can arise, you should use ifind-tail instead of ifind -- ifind-tail has no such ambiguity:

(cond ((ifind-tail pred lis) => (lambda (ipair) ...)) ; Handle (icar ipair)
      (else ...)) ; Search failed.
[procedure] (ifind-tail pred ilist)

Return the first ipair of ilist whose icar satisfies pred. If no ipair does, return false.

ifind-tail can be viewed as a general-predicate variant of the imember function.

Examples:

(ifind-tail even? (iq 3 1 37 -8 -5 0 0)) ;=> (-8 -5 0 0)
(ifind-tail even? (iq 3 1 37 -5))        ;=> #f

;; IMEMBER X LIS:
(ifind-tail (lambda (elt) (equal? x elt)) lis)

ifind-tail is essentially idrop-while, where the sense of the predicate is inverted: ifind-tail searches until it finds an element satisfying the predicate; idrop-while searches until it finds an element that doesn't satisfy the predicate.

[procedure] (itake-while pred ilist)

Returns the longest initial prefix of ilist whose elements all satisfy the predicate pred.

(itake-while even? (iq 2 18 3 10 22 9)) ;=> (2 18)
[procedure] (idrop-while pred ilist)

idrops the longest initial prefix of ilist whose elements all satisfy the predicate pred, and returns the rest of the ilist.

(idrop-while even? (iq 2 18 3 10 22 9)) ;=> (3 10 22 9)
[procedure] (ispan pred ilist)
[procedure] (ibreak pred ilist)

ispan splits the ilist into the longest initial prefix whose elements all satisfy pred, and the remaining tail. ibreak inverts the sense of the predicate: the tail commences with the first element of the input ilist that satisfies the predicate.

In other words: ispan finds the initial span of elements satisfying pred, and ibreak breaks the ilist at the first element satisfying pred.

ispan is equivalent to

(values (itake-while pred ilist)
        (idrop-while pred ilist))

(ispan even? (iq 2 18 3 10 22 9)) =>
  (2 18)
  (3 10 22 9)

(ibreak even? (iq 3 1 4 1 5 9)) =>
  (3 1)
  (4 1 5 9)
[procedure] (iany pred ilist1 ilist2 ...)

Applies the predicate across the ilists, returning true if the predicate returns true on any application.

If there are n ilist arguments ilist1 ... ilistn, then pred must be a procedure taking n arguments and returning a boolean result.

iany applies pred to the first elements of the ilisti parameters. If this application returns a true value, iany immediately returns that value. Otherwise, it iterates, applying pred to the second elements of the ilisti parameters, then the third, and so forth. The iteration stops when a true value is produced or one of the ilists runs out of values; in the latter case, iany returns #f. The application of pred to the last element of the ilists is a tail call.

Note the difference between ifind and ianyifind returns the element that satisfied the predicate; iany returns the true value that the predicate produced.

Like ievery, iany'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.

(iany integer? (iq a 3 b 2.7))   ;=> #t
(iany integer? (iq a 3.1 b 2.7)) ;=> #f
(iany < (iq 3 1 4 1 5)
        (iq 2 7 1 8 2))          ;=> #t
[procedure] (ievery pred ilist1 ilist2 ...)

Applies the predicate across the ilists, returning true if the predicate returns true on every application.

If there are n ilist arguments ilist1 ... ilistn, then pred must be a procedure taking n arguments and returning a boolean result.

ievery applies pred to the first elements of the ilisti parameters. If this application returns false, ievery immediately returns false. Otherwise, it iterates, applying pred to the second elements of the ilisti parameters, then the third, and so forth. The iteration stops when a false value is produced or one of the ilists runs out of values. In the latter case, ievery returns the true value produced by its final application of pred. The application of pred to the last element of the ilists is a tail call.

If one of the ilisti has no elements, ievery simply returns #t.

Like iany, ievery'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.

[procedure] (ilist-index pred ilist1 ilist2 ...)

Return the index of the leftmost element that satisfies pred.

If there are n ilist arguments ilist1 ... ilistn, then pred must be a function taking n arguments and returning a boolean result.

ilist-index applies pred to the first elements of the ilisti parameters. If this application returns true, ilist-index immediately returns zero. Otherwise, it iterates, applying pred to the second elements of the ilisti parameters, then the third, and so forth. When it finds a tuple of ilist elements that cause pred to return true, it stops and returns the zero-based index of that position in the ilists.

The iteration stops when one of the ilists runs out of values; in this case, ilist-index returns #f.

(ilist-index even? (iq 3 1 4 1 5 9))                  ;=> 2
(ilist-index < (iq 3 1 4 1 5 9 2 5 6) (iq 2 7 1 8 2)) ;=> 1
(ilist-index = (iq 3 1 4 1 5 9 2 5 6) (iq 2 7 1 8 2)) ;=> #f
[procedure] (imember x ilist [=])
[procedure] (imemq x ilist)
[procedure] (imemv x ilist)

These procedures return the first sub-ilist of ilist whose icar is x, where the sub-ilists of ilist are the non-empty ilists returned by (idrop ilist i) for i less than the length of ilist. If x does not occur in ilist, then #f is returned. imemq uses eq? to compare x with the elements of ilist, while imemv uses eqv?, and imember uses equal?.

(imemq 'a (iq a b c))           ;=>  (a b c)
(imemq 'b (iq a b c))           ;=>  (b c)
(imemq 'a (iq b c d))           ;=>  #f
(imemq (list 'a)
       (ilist 'b '(a) 'c))      ;=>  #f
(imember (list 'a)
         (ilist 'b '(a) 'c)))   ;=>  ((a) c)
(imemq 101 (iq 100 101 102))    ;=>  *unspecified*
(imemv 101 (iq 100 101 102))    ;=>  (101 102)

The comparison procedure is used to compare the elements ei of ilist to the key x in this way:

(= x ei) ; ilist is (E1 ... En)

That is, the first argument is always x, and the second argument is one of the ilist elements. Thus one can reliably find the first element of ilist that is greater than five with (imember 5 ilist <)

Note that fully general ilist searching may be performed with the ifind-tail and ifind procedures, e.g.

(ifind-tail even? ilist) ; Find the first elt with an even key.

Deleting

[procedure] (idelete x ilist [=])

idelete uses the comparison procedure =, which defaults to equal?, to find all elements of ilist that are equal to x, and deletes them from ilist. The dynamic order in which the various applications of {{}}=}} are made is not specified.

The ilist is not disordered — elements that appear in the result ilist occur in the same order as they occur in the argument ilist. The result may share a common tail with the argument ilist.

Note that fully general element deletion can be performed with the iremove procedures, e.g.:

;; idelete all the even elements from LIS:
(iremove even? lis)

The comparison procedure is used in this way: (= x ei). That is, x is always the first argument, and an ilist element is always the second argument. The comparison procedure will be used to compare each element of ilist exactly once; the order in which it is applied to the various ei is not specified. Thus, one can reliably remove all the numbers greater than five from an ilist with (idelete 5 ilist <)

[procedure] (idelete-duplicates ilist [=])

idelete-duplicates removes duplicate elements from the ilist argument. If there are multiple equal elements in the argument ilist, the result ilist 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 ilist -- idelete-duplicates does not disorder the ilist (hence it is useful for "cleaning up" immutable association lists).

The = parameter is used to compare the elements of the ilist; it defaults to equal?. If x comes before y in ilist, then the comparison is performed (= x y). The comparison procedure will be used to compare each pair of elements in ilist no more than once; the order in which it is applied to the various pairs is not specified.

Implementations of idelete-duplicates are allowed to share common tails between argument and result ilists -- for example, if the ilist argument contains only unique elements, it may simply return exactly this ilist.

Be aware that, in general, idelete-duplicates runs in time O(n^2) for n-element ilists. Uniquifying long ilists can be accomplished in O(n lg n) time by sorting the ilist to bring equal elements together, then using a linear-time algorithm to remove equal elements. Alternatively, one can use algorithms based on element-marking, with linear-time results.

(idelete-duplicates (iq a b a c a b c z)) ;=> (a b c z)

;; Clean up an ialist:
(idelete-duplicates (iq (a . 3) (b . 7) (a . 9) (c . 1))
                   (lambda (x y) (eq? (icar x) (icar y))))
    ;=> ((a . 3) (b . 7) (c . 1))

Immutable association lists

An "immutable association list" (or "ialist") is an ilist of ipairs. The icar of each ipair contains a key value, and the icdr contains the associated data value. They can be used to construct simple look-up tables in Scheme. Note that ialists are probably inappropriate for performance-critical use on large data; in these cases, immutable maps or some other alternative should be employed.

[procedure] (iassoc key ialist [=])
[procedure] (iassq key ialist)
[procedure] (iassv key ialist)

ialist must be an immutable association list — an ilist of ipairs. These procedures find the first ipair in ialist whose icar field is key, and returns that ipair. If no ipair in ialist has key as its icar, then #f is returned. iassq uses eq? to compare key with the icar fields of the ipairs in ialist, while iassv uses eqv? and iassoc uses equal?.

(define e (iq (a 1) (b 2) (c 3)))
(iassq 'a e)                               ;=>  (a 1)
(iassq 'b e)                               ;=>  (b 2)
(iassq 'd e)                               ;=>  #f
(iassq (ilist 'a) (iq ((a)) ((b)) ((c))))  ;=>  #f
(iassoc '(a) (ilist '((a)) '((b)) '((c)))) ;=>  ((a))
(iassq 5 (iq (2 3) (5 7) (11 13)))         ;=>  *unspecified*
(iassv 5 (iq (2 3) (5 7) (11 13)))         ;=>  (5 7)

The comparison procedure is used to compare the elements ei of ilist to the key parameter in this way:

(= key (icar ei)) ; ilist is (E1 ... En)

That is, the first argument is always key, and the second argument is one of the ilist elements. Thus one can reliably find the first entry of ialist whose key is greater than five with (iassoc 5 ialist <).

Note that fully general ialist searching may be performed with the ifind-tail and ifind procedures, e.g.

;; Look up the first association in ialist with an even key:
(ifind (lambda (a) (even? (icar a))) ialist)
[procedure] (ialist-cons key datum ialist)
(lambda (key datum ialist) (ipair (ipair key datum) ialist))

Construct a new ialist entry mapping key -> datum onto ialist.

[procedure] (ialist-delete key ialist [=])

ialist-delete deletes all associations from ialist with the given key, using key-comparison 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 ialist argument. The ialist is not disordered -- elements that appear in the result ialist occur in the same order as they occur in the argument ialist.

The comparison procedure is used to compare the element keys ki of ialist's entries to the key parameter in this way: (= key ki). Thus, one can reliably remove all entries of ialist whose key is greater than five with (ialist-delete 5 ialist <)

Replacement

These two procedures are analogues of the primitive side-effect operations on pairs, set-car! and set-cdr!.

[procedure] (replace-icar ipair object)
This procedure returns an ipair with object in the icar field and the icdr of

ipair in the icdr field.

[procedure] (replace-icdr ipair object)

This procedure returns an ipair with object in the icdr field and the icar of ipair in the icar field.

Conversion

These procedures convert between mutable and immutable pair structures.

[procedure] (pair->ipair pair)
[procedure] (ipair->pair ipair)

These procedures, which are inverses, return an ipair and a pair respectively that have the same (i)car and (i)cdr fields as the argument.

[procedure] (list->ilist flist)
[procedure] (ilist->list dlist)

These procedures return an ilist and a list respectively that have the same elements as the argument. The tails of dotted (i)lists are preserved in the result, which makes the procedures not inverses when the tail of a dotted ilist is a list or vice versa. The empty list is converted to itself.

It is an error to apply list->ilist to a circular list.

[procedure] (tree->itree obj)
[procedure] (itree->tree obj)

These procedures walk a tree of pairs or ipairs respectively and make a deep copy of it, returning an isomorphic tree containing ipairs or pairs respectively. The result may share structure with the argument. If the argument is not of the expected type, it is returned.

These procedures are not inverses in the general case. For example, a pair of ipairs would be converted by tree->itree to an ipair of ipairs, which if converted by itree->tree would produce a pair of pairs.

[procedure] (gtree->itree obj)
[procedure] (itree->gtree obj)

These procedures walk a generalized tree consisting of pairs, ipairs, or a combination of both, and make a deep copy of it, returning an isomorphic tree containing only ipairs or pairs respectively. The result may share structure with the argument. If the argument is neither a pair nor an ipair, it is returned.

Procedure application

This procedure allows a procedure to be applied to an ilist.

[procedure] (iapply procedure object ... ilist)

The iapply procedure is an analogue of apply whose last argument is an ilist rather than a list. It is equivalent to (apply procedure object ... (ilist->list ilist)), but may be implemented more efficiently.

Comparators

Currently not implemented in the sample implementation. Future versions should provide support for this through SRFI-128, as SRFI-114 has been deprecated in favour of this SRFI.

Repository

Github

Version History

1.4
Merges mailing list fixes for ilist=, ievery, and exports from Peter Lane.
1.3
Removes hardcoded .so extension in setup
1.2
Adds standard README.org to SRFIs
1.1
Packages egg without extraneous files
1.0
Initial release

License

Copyright (C) John Cowan (2014-2016). 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.