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The (ice-9 vlist) module provides an implementation of VList-based
hash lists (see VLists). VList-based hash lists, or vhashes, are an
immutable dictionary type similar to association lists that maps keys to
values. However, unlike association lists, accessing a value given its
key is typically a constant-time operation.
The VHash programming interface of (ice-9 vlist) is mostly the same as
that of association lists found in SRFI-1, with procedure names prefixed by
vhash- instead of alist- (see SRFI-1 Association Lists).
In addition, vhashes can be manipulated using VList operations:
(vlist-head (vhash-consq 'a 1 vlist-null)) ⇒ (a . 1) (define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null))) (define vh2 (vhash-consq 'c 3 (vlist-tail vh1))) (vhash-assq 'a vh2) ⇒ (a . 1) (vhash-assq 'b vh2) ⇒ #f (vhash-assq 'c vh2) ⇒ (c . 3) (vlist->list vh2) ⇒ ((c . 3) (a . 1))
However, keep in mind that procedures that construct new VLists
(vlist-map, vlist-filter, etc.) return raw VLists, not vhashes:
(define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq)) (vhash-assq 'a vh) ⇒ (a . 1) (define vl ;; This will create a raw vlist. (vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh)) (vhash-assq 'a vl) ⇒ ERROR: Wrong type argument in position 2 (vlist->list vl) ⇒ ((a . 1) (c . 3))
Return true if obj is a vhash.
Return a new hash list based on vhash where key is associated with
value, using hash-proc to compute the hash of key.
vhash must be either vlist-null or a vhash returned by a previous
call to vhash-cons. hash-proc defaults to hash (see hash procedure). With vhash-consq, the
hashq hash function is used; with vhash-consv the hashv
hash function is used.
All vhash-cons calls made to construct a vhash should use the same
hash-proc. Failing to do that, the result is undefined.
Return the first key/value pair from vhash whose key is equal to key
according to the equal? equality predicate (which defaults to
equal?), and using hash-proc (which defaults to hash) to
compute the hash of key. The second form uses eq? as the equality
predicate and hashq as the hash function; the last form uses eqv?
and hashv.
Note that it is important to consistently use the same hash function for
hash-proc as was passed to vhash-cons. Failing to do that, the
result is unpredictable.
Remove all associations from vhash with key, comparing keys with
equal? (which defaults to equal?), and computing the hash of
key using hash-proc (which defaults to hash). The second
form uses eq? as the equality predicate and hashq as the hash
function; the last one uses eqv? and hashv.
Again the choice of hash-proc must be consistent with previous calls to
vhash-cons.
Fold over the key/value elements of vhash in the given direction,
with each call to proc having the form (proc key value
result), where result is the result of the previous call to
proc and init the value of result for the first call
to proc.
Fold over all the values associated with key in vhash, with each
call to proc having the form (proc value result), where
result is the result of the previous call to proc and init the
value of result for the first call to proc.
Keys in vhash are hashed using hash are compared using equal?.
The second form uses eq? as the equality predicate and hashq as
the hash function; the third one uses eqv? and hashv.
Example:
(define vh (alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3)))) (vhash-fold* cons '() 'a vh) ⇒ (3 2 1) (vhash-fold* cons '() 'z vh) ⇒ (0)
Return the vhash corresponding to alist, an association list, using
hash-proc to compute key hashes. When omitted, hash-proc defaults
to hash.
Next: Hash Tables, Previous: Association Lists, Up: Data Types [Contents][Index]