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Pairs and lists are one of the most fundamental data structure in Scheme. Gauche core provides all standard list procedures, plus some useful procedures that are commonly supported in lots of implementations. If they are not enough, you can find more procedures in the modules described in R7RS lists and Combination library. See also Collection framework and Sequence framework for generic collection/sequence operations.

• Pair and null class: | ||

• Mutable and immutable pairs: | ||

• List predicates: | ||

• List constructors: | ||

• List accessors and modifiers: | ||

• Walking over lists: | ||

• Other list procedures: | ||

• Association lists: |

Next: Mutable and immutable pairs, Previous: Pairs and lists, Up: Pairs and lists [Contents][Index]

- Builtin Class:
**<list>** -
An abstract class represents lists. A parent class of

`<null>`

and`<pair>`

. Inherits`<sequence>`

.Note that a circular list is also an instance of the

`<list>`

class, while`list?`

returns false on the circular lists and dotted lists.(use srfi-1) (list? (circular-list 1 2)) ⇒ #f (is-a? (circular-list 1 2) <list>) ⇒ #t

- Builtin Class:
**<null>** -
A class of empty list.

`()`

is the only instance.

- Builtin Class:
**<pair>** -
A class of pairs.

Next: List predicates, Previous: Pair and null class, Up: Pairs and lists [Contents][Index]

A pair may be either mutable or immutable. You can desctructively
modify mutable pairs with `set-car!`

, `set-cdr!`

, or other
destructive procedures (usually they have `!`

at the end).
An error is signaled when you try to modify an immutable pair.

In Gauche, both type of pairs can be treated the same unless you try
to modify them. Both satisfies the predicate `pair?`

. If you
need to test specifically if a pair is immutable, use `ipair?`

(see List predicates).
The traditional constructor `cons`

creates a mutable pair; you
can create an immutable pair with `ipair`

(see List constructors).
More procedures that use immutable pairs are defined in
R7RS-large `scheme.ilist`

module (see R7RS immutable lists).

Note that quoted literals are immutable.
Old versions of Gauche wasn’t supported immutable pairs and quoted literal lists
were mutable. If your code accidentally mutate such pairs, you’ll get an
error. If you need to run such code without tracking down the error,
you can define the environment variable
`GAUCHE_MUTABLE_LITERALS`

.

Next: List constructors, Previous: Mutable and immutable pairs, Up: Pairs and lists [Contents][Index]

- Function:
**pair?***obj* [R7RS base] Returns

`#t`

if`obj`is a pair,`#f`

otherwise.

- Function:
**ipair?***obj* [R7RS ilist] Returns

`#t`

iff`obj`is an immutable pair,`#f`

otherwise.An immutable pair is indistinguishable from a mutable pair except using this predicate, or when you attempt to modify it. See Mutable and immutable pairs.

- Function:
**null?***obj* [R7RS base] Returns

`#t`

if`obj`is an empty list,`#f`

otherwise.

- Function:
**null-list?***obj* [R7RS list] Returns

`#t`

if`obj`is an empty list,`#f`

if`obj`is a pair. If`obj`is neither a pair nor an empty list, an error is signaled.This can be used instead of

`null?`

to check the end-of-list condition when you want to be more picky about non-proper lists.

- Function:
**list?***obj* [R7RS base] Returns

`#t`

if`obj`is a proper list,`#f`

otherwise. This function returns`#f`

if`obj`is a dotted or circular list.See also

`proper-list?`

,`circular-list?`

and`dotted-list?`

below.

- Function:
**proper-list?***x* [R7RS list] Returns

`#t`

fif x is a proper list, that is, a finite list terminated by`()`

.

- Function:
**circular-list?***x* [R7RS list] Returns

`#t`

if x is a circular list. A list is circular if you follow`cdr`

of the pairs you’ll eventually get to a pair you already visited. It doesn’t necessary that the head of the list`x`is a part of the circle. A list isn’t circular by the cycle that involves`car`of the paris.

- Function:
**dotted-list?***x* [R7RS list] Returns

`#t`

if x is a finite, non-nil-terminated list. This includes non-pair, non-() values (e.g. symbols, numbers), which are considered to be dotted lists of length 0.

Next: List accessors and modifiers, Previous: List predicates, Up: Pairs and lists [Contents][Index]

- Function:
**cons***obj1 obj2* [R7RS base] Constructs a mutable pair of

`obj1`and`obj2`and returns it.(cons 'a 'b) ⇒ (a . b)

- Function:
**ipair***obj1 obj2* [R7RS ilist] Constructs an immutable pair of

`obj1`and`obj2`and returns it.(ipair 'a 'b) ⇒ (a . b)

- Function:
**make-list***len :optional fill* [R7RS base] Makes a proper list of length

`len`. If optional argument`fill`is provided, each element is initialized by it. Otherwise each element is undefined.(make-list 5 #t) ⇒ (#t #t #t #t #t)

- Function:
**list***obj …* [R7RS base] Makes a list, whose elements are

`obj`….(list 1 2 3) ⇒ (1 2 3) (list) ⇒ ()

- Function:
**ilist***obj …* [R7RS ilist] Makes a list, whose elements are

`obj`…, and which consists of immutable pairs.(ilist 1 2 3) ⇒ (1 2 3) (ilist) ⇒ () (list-set! (ilist 1 2 3) 1 'a) ⇒ ERROR: Attempt to modify an immutable pair: (2 3)

- Function:
**list****obj1 obj2 …* - Function:
**cons****obj1 obj2 …* [R7RS list] Like

`list`

, but the last argument becomes cdr of the last pair. Two procedures are exactly the same. Gauche originally had`list*`

, and SRFI-1 (R7RS`(scheme list)`

)defines`cons*`

.(list* 1 2 3) ⇒ (1 2 . 3) (list* 1) ⇒ 1

- Function:
**list-copy***list* [R7RS base] Shallow copies

`list`. If`list`is circular, an error is thrown. (Detecting circular list is Gauche’s extension; R7RS allows the procedure to diverge.)

- Function:
**iota***count :optional (start 0) (step 1)* [R7RS list] Returns a list of

`count`numbers, starting from`start`, increasing by`step`.`Count`must be a nonnegative integer. If both`start`and`step`are exact, the result is a list of exact numbers; otherwise, it is a list of inexact numbers.(iota 5) ⇒ (0 1 2 3 4) (iota 5 1 3/7) ⇒ (1 10/7 13/7 16/7 19/7) (iota 5 0 -0.1) ⇒ (0 -0.1 -0.2 -0.3 -0.4)

This creates a list eagerly. If the list is short it is fast enough, but if you want to count tens of thousands of numbers, you may want to do so lazily. See

`liota`

(see Lazy sequences).

- Macro:
**cond-list***clause …* Construct a list by conditionally adding entries. Each

`clause`has a test and expressions. When its test yields true, the result of associated expression is used to construct the resulting list. When the test yields false, nothing is inserted.`Clause`must be either one of the following form:`(`

`test``expr`…)`Test`is evaluated, and when it is true,`expr`… are evaluated, and the return value becomes a part of the result. If no`expr`is given, the result of`test`is used if it is not false.`(`

`test`=>`proc`)`Test`is evaluated, and when it is true,`proc`is called with the value, and the return value is used to construct the result.`(`

`test`@`expr`…)Like

`(test expr …)`

, except that the result of the last`expr`must be a list, and it is spliced into the resulting list, like unquote-splicing.`(`

`test`=> @`proc`)Like

`(test => proc)`

, except that the result of`proc`must be a list, and and it is spliced into the resulting list, like unquote-splicing.

(let ((alist '((x 3) (y -1) (z 6)))) (cond-list ((assoc 'x alist) 'have-x) ((assoc 'w alist) 'have-w) ((assoc 'z alist) => cadr))) ⇒ (have-x 6) (let ((x 2) (y #f) (z 5)) (cond-list (x @ `(:x ,x)) (y @ `(:y ,y)) (z @ `(:z ,z)))) ⇒ (:x 2 :z 5)

Next: Walking over lists, Previous: List constructors, Up: Pairs and lists [Contents][Index]

- Function:
**car***pair* - Function:
**cdr***pair* [R7RS base] Returns car and cdr of

`pair`, respectively.

- Function:
**set-car!***pair obj* - Function:
**set-cdr!***pair obj* [R7RS base] Modifies car and cdr of

`pair`, by`obj`, respectively.Note:

`(setter car)`

≡`set-car!`

, and`(setter cdr)`

≡`set-cdr!`

.

- Function:
**caar***pair* - Function:
**cadr***pair* -
…

- Function:
**cdddar***pair* - Function:
**cddddr***pair* [R7RS base][R7RS cxr]

`caar`

≡`(car (car x))`

,`cadr`

≡`(car (cdr x))`

, and so on.In R7RS, more than two-level of accessors are defined in the

`(scheme cxr)`

library.The corresponding setters are also defined.

(let ((x (list 1 2 3 4 5))) (set! (caddr x) -1) x) ⇒ (1 2 -1 4 5)

- Function:
**length***list* [R7RS base] Returns the length of a proper list

`list`. If`list`is a dotted list, an error is signaled. If`list`is a circular list, this function diverges.

- Function:
**length+***x* [R7RS list] If

`x`is a proper list, returns its length. For all other`x`, including a circular list, it returns`#f`

.

- Function:
**length=?***x k* - Function:
**length<?***x k* - Function:
**length<=?***x k* - Function:
**length>?***x k* - Function:
**length>=?***x k* Returns

`#t`

iff`x`is a (possibly improper) list whose length is equal to, less than, less than or equal to, greater than, or greater than or equal to an exact integer`k`, respectively. This procedure only follows the list up to the`k`items, so it doesn’t realize elements of lazy sequence more than needed (See Lazy sequences, for the lazy sequences).Dotted lists and circular lists are allowed. For the dotted list, the cdr of the last pair isn’t counted; that is, a non-pair object has length 0, and

`(a . b)`

has length 1. A circular list is treated as if it has infinite length.(length<=? '(a b) 2) ⇒ #t (length<=? '(a b) 1) ⇒ #f (length<=? '() 0) ⇒ #t ;; dotted list cases (length<=? 'a 0) ⇒ #t (length<=? '(a . b) 0) ⇒ #f (length<=? '(a . b) 1) ⇒ #t

NB: The name of these procedures might be misleading, for other procedures with the name

`something<=?`

etc. usually takes objects of the same type. We don’t have any better idea now, unfortunately.

- Function:
**take***x i* - Function:
**drop***x i* [R7RS list]

`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:(take '(1 2 3 . d) 2) => (1 2) (drop '(1 2 3 . d) 2) => (3 . d) (drop '(1 2 3 . d) 3) => d

`drop`

is exactly equivalent to performing`i`cdr operations on`x`. The returned value shares a common tail with`x`. On the other hand, take always allocates a new list for result if the argument is a list of non-zero length.An error is signaled if

`i`is past the end of list`x`. See`take*`

and`drop*`

below for more tolerant version.For generic subsequence extraction from any sequence, see

`subseq`

in Slicing sequence.

- Function:
**take****list k :optional (fill? #f) (padding #f)* - Function:
**drop****list k* More tolerant version of

`take`

and`drop`

. They won’t raise an error even if`k`is larger than the size of the given list.If the list is shorter than

`k`elements,`take*`

returns a copy of`list`by default. If`fill?`is true,`padding`is added to the result to make its length`k`.On the other hand,

`drop*`

just returns an empty list when the input list is shorter than`k`elements.(take* '(a b c d) 3) ⇒ (a b c) (take* '(a b c d) 6) ⇒ (a b c d) (take* '(a b c d) 6 #t) ⇒ (a b c d #f #f) (take* '(a b c d) 6 #t 'z) ⇒ (a b c d z z) (drop* '(a b c d) 3) ⇒ (d) (drop* '(a b c d) 5) ⇒ ()

Note: For generic subsequence extraction from any sequence, see

`subseq`

in Slicing sequence.

- Function:
**take-right***lis k* - Function:
**drop-right***lis k* [R7RS list]

`take-right`

returns the last`k`elements of`lis`.`drop-right`

returns all but the last`k`elements of`lis`.(take-right '(a b c d e) 2) => (d e) (drop-right '(a b c d e) 2) => (a b c)

`lis`may be any finite list.(take-right '(1 2 3 . d) 2) => (2 3 . d) (drop-right '(1 2 3 . d) 2) => (1) (take-right '(1 2 3 . d) 0) => d (drop-right '(1 2 3 . d) 0) => (1 2 3)

`take-right`

’s return value always shares a common tail with`lis`.`drop-right`

always allocates a new list if the argument is a list of non-zero length.An error is signaled if

`k`is larger than the length of`lis`. See`take-right*`

and`drop-right*`

below, for more tolerant version.

- Function:
**take-right****list k :optional (fill? #f) (padding #f)* - Function:
**drop-right****list k* Like

`take*`

and`drop*`

, but counts from right of`list`. If`list`is shorter than`k`elements, they won’t raise an error. Instead,`drop-right*`

just returns an empty list, and`take-right*`

returns`list`itself by default. If`fill?`is true for`take-right*`

,`padding`is added on the left of the result to make its length`k`. The result still shares the`list`.

- Function:
**take!***lis k* - Function:
**drop-right!***lis k* [R7RS list] Linear update variants of

`take`

and`drop-right`

. Those procedures may destructively modifies`lis`.If

`lis`is circular,`take!`

may return a list shorter than expected.

- Function:
**list-tail***list k :optional fallback* [R7RS base] Returns

`k`-th cdr of`list`.`list`can be a proper, dotted or circular list. (If`list`is a dotted list, its last`cdr`

is simply ignored).If

`k`is negative or larger than the length of`list`, the behavior depends on whether the optional`fallback`argument is given or not. If`fallback`is given, it is returned. Otherwise, an error is signaled.

- Function:
**list-ref***list k :optional fallback* [R7RS+] Returns

`k`-th element of`list`.`list`can be a proper, dotted or circular list.By default,

`list-ref`

signals an error if`k`is negative, or greater than or equal to the length of`list`. However, if an optional argument`fallback`is given, it is returned for such case. This is an extension of Gauche.

- Function:
**list-set!***list k v* [R7RS base] Modifies the

`k`-th element of a`list`by`v`. It is an error unless`k`is an exact integer between 0 and one minus the length of`k`. If`list`is immutable, no error is signalled but the behavior is undefined.

- Function:
**last-pair***list* [R7RS list] Returns the last pair of

`list`.`list`can be a proper or dotted list.(last-pair '(1 2 3)) ⇒ (3) (last-pair '(1 2 . 3)) ⇒ (2 . 3) (last-pair 1) ⇒

*error*

- Function:
**last***pair* [R7RS list] Returns the last element of the non-empty, finite list

`pair`. It is equivalent to`(car (last-pair pair))`

.(last '(1 2 3)) ⇒ 3 (last '(1 2 . 3)) ⇒ 2

- Function:
**split-at***x i* - Function:
**split-at!***x i* [R7RS list]

`split-at`

splits the list`x`at index`i`, returning a list of the first`i`elements, and the remaining tail.(split-at '(a b c d e) 2) ⇒ (a b) (c d e)

`split-at!`

is the linear-update variant. It may destructively modifies`x`to produce the result.

- Function:
**split-at****list k :optional (fill? #f) (padding #f)* More tolerant version of

`split-at`

. Returns the results of`take*`

and`drop*`

.`(split-at* '(a b c d) 6 #t 'z) ⇒ (a b c d z z) and ()`

- Function:
**slices***list k :optional fill? padding* Splits

`list`into the sublists (slices) where the length of each slice is`k`. If the length of`list`is not a multiple of`k`, the last slice is dealt in the same way as`take*`

; that is, it is shorter than`k`by default, or added`padding`if`fill?`is true.(slices '(a b c d e f g) 3) ⇒ ((a b c) (d e f) (g)) (slices '(a b c d e f g) 3 #t 'z) ⇒ ((a b c) (d e f) (g z z))

- Function:
**intersperse***item list* Inserts

`item`between elements in the`list`. (The order of arguments is taken from Haskell’s intersperse).(intersperse '+ '(1 2 3)) ⇒ (1 + 2 + 3) (intersperse '+ '(1)) ⇒ (1) (intersperse '+ '()) ⇒ ()

Next: Other list procedures, Previous: List accessors and modifiers, Up: Pairs and lists [Contents][Index]

- Function:
**map***proc list1 list2 …* [R7RS+] Applies

`proc`for each element(s) of given list(s), and returns a list of the results. R7RS doesn’t specify the application order of`map`

, but Gauche guarantees`proc`is always applied in order of the list(s). Gauche’s`map`

also terminates as soon as one of the list is exhausted.(map car '((a b) (c d) (e f))) ⇒ (a c e) (map cons '(a b c) '(d e f)) ⇒ ((a . d) (b . e) (c . f))

Note that the

`gauche.collection`

module (see Collection framework) extends`map`

to work on any type of collection.

- Function:
**append-map***f clist1 clist2 …* - Function:
**append-map!***f clist1 clist2 …* [R7RS list] Functionally equivalent to the followings, though a bit more efficient:

(apply append (map

`f``clist1``clist2`…)) (apply append! (map`f``clist1``clist2`…))At least one of the list arguments must be finite.

- Function:
**map****proc tail-proc list1 list2 …* Like

`map`

, except that`tail-proc`is applied to the`cdr`

of the last pair in the argument(s) to get the`cdr`

of the last pair of the result list. This procedure allows improper list to appear in the arguments. If a single list is given,`tail-proc`always receives a non-pair object.(map* - / '(1 2 3 . 4)) ⇒ (-1 -2 -3 . 1/4) (define (proper lis) (map* values (lambda (p) (if (null? p) '() (list p))) lis)) (proper '(1 2 3)) ⇒ (1 2 3) (proper '(1 2 3 . 4)) ⇒ (1 2 3 4)

If more than one list are given, the shortest one determines how

`tail-proc`is called. When`map*`

reaches the last pair of the shortest list,`tail-proc`is called with cdrs of the current pairs.(map* + vector '(1 2 3 4) '(1 2 . 3)) ⇒ (2 4 . #((3 4) 3))

Note: The name

`map*`

is along the line of`list*`

/`cons*`

that can produce improper list (See List constructors, see R7RS lists).

- Function:
**for-each***proc list1 list2 …* [R7RS base] Applies

`proc`for each element(s) of given list(s) in order. The results of`proc`are discarded. The return value of`for-each`

is undefined. When more than one list is given,`for-each`

terminates as soon as one of the list is exhausted.Note that the

`gauche.collection`

module (see Collection framework) extends`for-each`

to work on any type of collection.

- Function:
**fold***kons knil clist1 clist2 …* [R7RS list] The fundamental list iterator. When it is given a single list

`clist1`= (`e1``e2`…`en`), then this procedure returns(

`kons``en`… (`kons``e2`(`kons``e1``knil`)) … )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. At least one of the list arguments must be finite.Examples:

(fold + 0 '(3 1 4 1 5 9)) ⇒ 23 ;sum up the elements (fold cons '() '(a b c d e)) ⇒ (e d c b a) ;reverse (fold cons* '() '(a b c) '(1 2 3 4 5)) ⇒ (c 3 b 2 a 1) ;n-ary case

- Function:
**fold-right***kons knil clist1 clist2 …* [R7RS list] The fundamental list recursion operator. When it is given a single list

`clist1`= (`e1``e2`…`en`), then this procedure returns(

`kons``e1`(`kons``e2`… (`kons``en``knil`)))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. At least one of the list arguments must be finite.Examples:

(fold-right cons '() '(a b c d e)) ⇒ (a b c d e) ;copy list (fold-right cons* '() '(a b c) '(1 2 3 4 5)) ⇒ (a 1 b 2 c 3) ;n-ary case

- Function:
**fold-left***snok knil clist1 clist2 …* [R6RS] This is another variation of left-associative folding. When it is given a single list

`clist1`= (`e1``e2`…`en`), then this procedure returns:(

`snok`(… (`snok`(`snok``knil``e1`)`e2`) …)`en`)Compare this with

`fold`

above; association is the same, but the order of arguments passed to the procedure`snok`is reversed from the way arguments are passed to`kons`in`fold`

. If`snok`is commutative,`fold`

and`fold-left`

produces the same result.(fold-left + 0 '(1 2 3 4 5) ⇒ 15 (fold-left cons 'z '(a b c d)) ⇒ ((((z . a) . b) . c) . d) (fold-left (^[a b] (cons b a)) 'z '(a b c d)) ⇒ (a b c d z)

If more than one lists are given,

`snok`is called with the current seed value`knil`and each corresponding element of the input lists`clist1`

`clist2`

….(fold-left list 'z '(a b c) '(A B C)) ⇒ (((z a A) b B) c C)

Note: Most functional languages have left- and right- associative fold operations, which correspond to

`fold-left`

and`fold-right`

, respectively. (e.g. Haskell’s`foldl`

and`foldr`

). In Scheme,`SRFI-1`

first introduced`fold`

and`fold-right`

. R6RS introduced`fold-left`

. (However, in R6RS the behavior is undefined if the lengths of`clist1`

`clist2`

… aren’t the same, while in Gauche`fold-left`

terminates as soon as any one of the lists terminates.)

- Function:
**reduce***f ridentity list* - Function:
**reduce-right***f ridentity list* [R7RS list] Variant of

`fold`

and`fold-right`

.`f`must be a binary operator, and`ridentity`is the value such that for any value`x`that is valid as`f`’s input,(f x ridentity) ≡ x

These functions effectively do the same thing as

`fold`

or`fold-right`

, respectively, but omit the first application of`f`to`ridentity`, using the above nature. So`ridentity`is used only when`list`is empty.

- Function:
**filter***pred list* - Function:
**filter!***pred list* [R7RS list] A procedure

`pred`is applied on each element of`list`, and a list of elements that`pred`returned true on it is returned.(filter odd? '(3 1 4 5 9 2 6)) ⇒ (3 1 5 9)

`filter!`

is the linear-update variant. It may destructively modifies`list`to produce the result.

- Function:
**filter-map***f clist1 clist2 …* [R7RS list] Like

`map`

, but only true values are saved. At least one of the list arguments must be finite.(filter-map (lambda (x) (and (number? x) (* x x))) '(a 1 b 3 c 7)) ⇒ (1 9 49)

- Function:
**remove***pred list* - Function:
**remove!***pred list* [R7RS list] A procedure

`pred`is applied on each element of`list`, and a list of elements that`pred`returned false on it is returned.(remove odd? '(3 1 4 5 9 2 6)) ⇒ (4 2 6)

`remove!`

is the linear-update variant. It may destructively modifies`list`to produce the result.

- Function:
**find***pred clist* [R7RS list] Applies

`pred`for each element of`clist`, from left to right, and returns the first element that`pred`returns true on. If no element satisfies`pred`,`#f`

is returned.

- Function:
**find-tail***pred clist* [R7RS list] Applies

`pred`for each element of`clist`, from left to right, and when`pred`returns a true value, returns the pair whose car is the element. If no element satisfies`pred`,`#f`

is returned.

- Function:
**any***pred clist1 clist2 …* [R7RS list] Applies

`pred`across each element of`clist`s, and returns as soon as`pred`returns a non-false value. The return value of`any`

is the non-false value`pred`returned. If`clist`s are exhausted before`pred`returns a non-false value,`#f`

is returned.

- Function:
**every***pred clist1 clist2 …* [R7RS list] Applies

`pred`across each element of`clist`s, and returns`#f`

as soon as`pred`returns`#f`

. If all application of`pred`return a non-false value,`every`

returns the last result of the applications.

- Function:
**count***pred clist1 clist2 …* [R7RS list] A procedure

`pred`is applied to the`n`-th element of given lists, from`n`is zero to the length of the the shortest finite list in the given lists, and the count of times`pred`returned true is returned.(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) (circular-list 1 10)) ⇒ 2

- Function:
**delete***x list :optional elt=* - Function:
**delete!***x list :optional elt=* [R7RS list] Equivalent to

(remove (lambda (y) (elt= x y)) list) (remove! (lambda (y) (elt= x y)) list)

The comparison procedure,

`elt=`, defaults to`equal?`

.

- Function:
**delete-duplicates***list :optional elt=* - Function:
**delete-duplicates!***list :optional elt=* [R7RS list] Removes duplicate elements from

`list`. If there are multiple equal elements in`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. The comparison procedure,`elt=`, defaults to`equal?`

.

Next: Association lists, Previous: Walking over lists, Up: Pairs and lists [Contents][Index]

- Function:
**append***list …* [R7RS base] Returns a list consisting of the elements of the first

`list`followed by the elements of the other lists. The resulting list is always newly allocated, except that it shares structure with the last list argument. The last argument may actually be any object; an improper list results if the last argument is not a proper list.

- Function:
**append!***list …* [R7RS list] Returns a list consisting of the elements of the first

`list`followed by the elements of the other lists. The cells in the lists except the last one may be reused to construct the result. The last argument may be any object.

- Function:
**concatenate***list-of-lists* - Function:
**concatenate!***list-of-lists!* [R7RS list] Equivalent to

`(apply append`

and`list-of-lists`)`(apply append!`

, respectively, but this can be a bit efficient by skipping overhead of`list-of-lists`)`apply`

.

- Function:
**reverse***list :optional (tail '())* - Function:
**reverse!***list :optional (tail '())* [R7RS+] Returns a list consisting of the elements of

`list`in the reverse order. While`reverse`

always returns a newly allocated list,`reverse!`

may reuse the cells of`list`. Even`list`is destructively modified by`reverse!`

, you should use its return value, for the first cell of`list`may not be the first cell of the returned list.If an optional argument

`tail`is given, it becomes the tail of the returned list (`tail`isn’t copied). It is useful in the idiom to prepend the processed results on top of already existing results.(reverse '(1 2 3 4 5)) ⇒ (5 4 3 2 1) (reverse '(1 2 3) '(a b)) ⇒ (3 2 1 a b)

The

`tail`argument is Gauche’s extension, and it isn’t in the traditional Scheme’s`reverse`

. The rationale is the following correspondence:(reverse xs) ≡ (fold cons xs '()) (reverse xs tail) ≡ (fold cons xs tail)

- Function:
**append-reverse***rev-head tail* - Function:
**append-reverse!***rev-head tail* [R7RS list] Equivalent to the two-argument

`reverse`

and`reverse!`

. Provided for srfi-1 (R7RS`(scheme list)`

) compatibility.

- Function:
**memq***obj list* - Function:
**memv***obj list* - Function:
**member***obj list :optional obj=* [R7RS base] Searches

`obj`in the`list`. If`n`

-th element of`list`equals to`obj`(in the sense of`eq?`

for`memq`

,`eqv?`

for`memv`

, and`equal?`

for`member`

),`(list-tail`

is returned. Otherwise,`list``n`)`#f`

is returned.If the optional

`obj=`argument of`member`

is given, it is used as a equivalence predicate instead of`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 (memv 101 '(100 101 102)) ⇒ (101 102)

Previous: Other list procedures, Up: Pairs and lists [Contents][Index]

- Function:
**acons***obj1 obj2 obj3* Returns

`(cons (cons`

. Useful to put an entry at the head of an associative list.`obj1``obj2`)`obj3`)(This procedure is defined in SRFI-1 (R7RS

`(scheme list)`

) as`alist-cons`

; see R7RS lists).(acons 'a 'b '((c . d))) ⇒ ((a . b) (c . d))

- Function:
**alist-copy***alist* [R7RS list] Returns a fresh copy of

`alist`. The spine of`alist`and each cell that points a key and a value is copied.(define a (list (cons 'a 'b) (cons 'c 'd))) a ⇒ ((a . b) (c . d)) (define b (alist-copy a)) b ⇒ ((a . b) (c . d)) (set-cdr! (car a) 'z) a ⇒ ((a . z) (c . d)) b ⇒ ((a . b) (c . d))

- Function:
**assq***obj list* - Function:
**assv***obj list* - Function:
**assoc***obj list :optional key=* [R7RS base] Each element in

`list`should be a pair (Gauche ignores non-pair element in`list`, but other R7RS implementation may raise an error, so be aware of it when you’re writing a portable code). These procedures search a pair whose car matches`obj`(in the sense of`eq?`

for`assq`

,`eqv?`

for`assv`

, and`equal?`

for`assoc`

) from left to right, and return the leftmost matched pair if any. If no pair matches, these return`#f`

.If the optional argument of

`assoc`

is given, it is called instead of`equal?`

to check the equivalence of`obj`and each key.

- Function:
**alist-delete***key alist :optional key=* - Function:
**alist-delete!***key alist :optional key=* [R7RS list] Deletes all cells in

`alist`whose key is the same as`key`. Comparison is done by a procedure`key=`. The default is`eqv?`

.The linear-update version

`alist-delete!`

may or may not modify`alist`.

- Function:
**rassoc***key alist :optional eq-fn* - Function:
**rassq***key alist* - Function:
**rassv***key alist* Reverse associations—given

`key`is matched to the*cdr*of each element in`alist`, instead of the*car*. Handy to realize bidirectional associative list.`Rassoc`

takes an optional comparison function, whose default is`equal?`

.`Rassq`

and`rassv`

uses`eq?`

and`eqv?`

.

- Function:
**assoc-ref***alist key :optional default eq-fn* - Function:
**assq-ref***alist key :optional default* - Function:
**assv-ref***alist key :optional default* These procedures provide the access to the assoc list symmetric with other

`*-ref`

procedures. (Note that the argument order is different from`assoc`

,`assq`

and`assv`

–`*-ref`

procedures take a container first, and an item second.)This captures the common pattern of alist access:

(assoc-ref alist key default eq-fn) ≡ (cond [(assoc key alist eq-fn) => cdr] [else default])))

If

`default`is omitted,`#f`

is used.`Assoc-ref`

takes an optional comparison function`eq-fn`, whose default is`equal?`

.`Assq-ref`

and`assv-ref`

uses`eq?`

and`eqv?`

, respectively.

- Function:
**rassoc-ref***alist key :optional default eq-fn* - Function:
**rassq-ref***alist key :optional default* - Function:
**rassv-ref***alist key :optional default* Reverse association version of

`assoc-ref`

.(rassoc-ref alist key default eq-fn) ≡ (cond ((rassoc key alist eq-fn) => car) (else default))))

The meanings of optional arguments are the same as

`assoc-ref`

.

- Function:
**assoc-set!***alist key val :optional eq-fn* - Function:
**assq-set!***alist key val* - Function:
**assv-set!***alist key val* Returns an alist who has

`(key . val)`

pair added to the`alist`

. If`alist`

already has an element with`key`, the element’s*cdr*is destructively modified for`val`. If`alist`doesn’t have an element with`key`, a new pair is created and appended in front of`alist`; so you should use the return value to guarantee`key`-`val`pair is added.`Assoc-set!`

takes optional comparison function`eq-fn`, whose default is`equal?`

.`Assq-set!`

and`assv-set!`

uses`eq?`

and`eqv?`

, respectively.

- Function:
**assoc-adjoin***alist key val :optional eq-fn* If

`alist`contains an entry with`key`, returns a new associative list where the value of the`key`is replaced for`val`. The order of entries in`alist`is preserved. If`alist`doesn’t contain the entry, it returns`(acons`

.`key``val``alist`)The original

`alist`is left unmodified. The returned associative list may share a part of its tail with the original`alist`, however.The optional

`eq-fn`argument is a procedure with two arguments to be used to compare the keys; the default is`equal?`

.Note the order of arguments; we have

`alist`first, just as`assoc-ref`

and`assoc-set!`

, and other`-adjoin`

procedures. It is not the same as`alist-delete`

and`assoc`

, which takes the key first.

- Function:
**assoc-update-in***alist keys proc :optional default eq-fn* This procedure allows to update a nested associative list. The

`alist`argument is a (possibly nested) associative list,`keys`are a list of keys, and`proc`is a procedure that takes one argument. First, the keys are looked up recursively in`alist`; then its value is passed to`proc`. The return value is a new (nested) associative list where the value pointed by`keys`is replaced with the return value of`proc`.(assoc-update-in '((a (b . 1) (c . 2))) '(a c) (cut + <> 1)) ⇒ ((a (b . 1) (c . 3))

The order of entries are preserved. The original

`alist`is left unmodified, but the returned value may share a part of the structure with`alist`.If

`alist`doesn’t have the entry specified by`keys`, a new entry is added. A new entry is added at the beginning of the sequence where specified key didn’t exist.(assoc-update-in '((a (b . 1) (c . 2))) '(a d e) (^_ 99)) ⇒ ((a (d (e . 99)) (b . 1) (c . 3)))

The

`default`argument is passed to`proc`when there’s no entry with specified keys. If omitted,`#f`

is assumed.The optional

`eq-fn`argument is a procedure with two arguments to be used to compare the keys; the default is`equal?`

.Note the order of arguments; we have

`alist`first, just as`assoc-ref`

and`assoc-set!`

, and other`-adjoin`

procedures. It is not the same as`alist-delete`

and`assoc`

, which takes the key first.Note: For destructively updating general nested aggregate structures, setter of

`~`

is handy (see Universal accessor). You can modify an entry in a hashtable in a vector in a list, for example. Associative list is a bit special, since you can’t distinguish it from lists (thus`~`

can’t be used), and it is mostly used in functional way. So we added a special update procedure.

Previous: Other list procedures, Up: Pairs and lists [Contents][Index]