[pro] "fhash"

[Sam Steingold]

> * Daniel Weinreb <qyj at tbbtyr.pbz> [2011-06-30 18:12:02 -0400]:
>
> Just for the record, I think extensible sequences would be really great!

Just for the record, CLISP has had extensible sequences since forever
(far predating my involvement).
See clisp/src/defseq.lisp which defines the standard sequences (vectors,
lists et al); the infrastructure is in C file clisp/src/sequence.d:

/* O(seq_types) contains a list of type descriptors for sequences.
 These are simple-vectors of length 16, containing:
  SEQ-TYPE        ; the type of the sequence, usually a symbol
 Access functions:
  SEQ-INIT
  SEQ-UPD
  SEQ-ENDTEST
  SEQ-FE-INIT
  SEQ-FE-UPD
  SEQ-FE-ENDTEST
  SEQ-ACCESS
  SEQ-ACCESS-SET
  SEQ-COPY
  SEQ-LENGTH
  SEQ-MAKE
  SEQ-ELT
  SEQ-SET-ELT
  SEQ-INIT-START
  SEQ-FE-INIT-END

Explanation of the functions SEQ-XXX:

A "pointer" is something, that can step through a sequence.
There are pointers, that move from left to right;
  they are created with INIT or INIT-START, copied with COPY,
    UPD to advance one step,
    ENDTEST for testing, if they have reached the end of the Sequence,
    ACCESS  for fetching the element, which is pointed to by the pointer,
    ACCESS-SET for setting the element, which is pointed to by the pointer.
There are also pointers, that move from right to left;
  they are created with FE-INIT or FE-INIT-END, copied with COPY,
    FE-UPD for moving them one step to the left,
    FE-ENDTEST for testing, if they have reached the end of the Sequence,
    ACCESS for fetching the element, which is pointed to by the pointer.
  For them, ACCESS-SET does not work.

Movement operations:
INIT          (lambda (seq) ...) -> pointer
              returns the leftmost pointer of SEQ.
UPD           (lambda (seq pointer) ...) -> pointer
              returns a pointer to the adjacent neighbor at the right.
              SEQ-UPD can assume, that the right border of
              SEQ is not stepped over.
ENDTEST       (lambda (seq pointer) ...) -> bool
              tests, if this pointer is at the right end of SEQ.
The same "FROM END" :
FE-INIT       (lambda (seq) ...) -> pointer
              returns the rightmost pointer of SEQ.
FE-UPD        (lambda (seq pointer) ...) -> pointer
              returns a pointer to the adjacent neighbor at the left.
              SEQ-FE-UPD can assume, that the left border of
              SEQ is not stepped over.
FE-ENDTEST    (lambda (seq pointer) ...) -> bool
              tests, if this pointer is at the left end of SEQ.
Access via pointer:
ACCESS        (lambda (seq pointer) ...) -> value
              returns the element in SEQ the pointer is pointing to.
ACCESS-SET    (lambda (seq pointer value) ...) ->
              sets the element where the pointer is pointing to in SEQ, to the
              specified value. Works only for pointers that move from left to
              right!
COPY          (lambda (pointer) ...) -> pointer
              returns a copy of the Pointer to SEQ (because UPD and FE-UPD
              can operate destructively on the pointers)
Total length:
LENGTH        (lambda (seq) ...) -> size
              returns the (active) length of the Sequence SEQ.
MAKE          (lambda (size) ...) -> sequence
              returns a newly allocated, empty sequence, that has the type
              SEQ-TYPE and the specified length.
Access via index (usually more inefficient than via pointer):
ELT           (lambda (seq index) ...) -> value
              returns (ELT SEQ index)
SET-ELT       (lambda (seq index value) ...) ->
              sets (ELT SEQ index) to value.
INIT-START    (lambda (seq index) ...) -> pointer
              returns a pointer which moves in SEQ from left to right
              from Position index. Must execute the Range-test by itself.
FE-INIT-END   (lambda (seq index) ...) -> pointer
              returns a pointer which moves in SEQ from right to left
              from Position index. Must execute the Range-test by itself.
*/

of course, the presence of index accessors prevents things like hash
tables, trees and maps from being considered sequences...

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