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| -rw-r--r-- | txr.1 | 198 |
1 files changed, 99 insertions, 99 deletions
@@ -29308,6 +29308,105 @@ yields .code nil because the set of integers beginning with 0 isn't empty. +.coNP Function @ nullify +.synb +.mets (nullify << iterable ) +.syne +.desc +The +.code nullify +function returns +.code nil +if +.meta iterable +denotes an empty sequence. +Otherwise, if +.meta iterable +is not an empty sequence, or isn't a sequence, then +.meta iterable +itself is returned. + +If +.meta iterable +is a structure object which supports the +.code nullify +method, then that method is called. If it returns +.code nil +then +.code nil +is returned. If the +.code nullify +method returns a substitute object other than the +.meta iterable +object itself, then +.code nullify +is invoked on that returned substitute object. + +Note: the +.code nullify +function is a helper to support unoptimized generic +traversal of sequences. Thanks to the generalized behavior of +.codn cdr , +non-list sequences can be traversed using +.codn cdr , +similarly to proper lists, by checking for +.code cdr +returning the terminating value +.codn nil . +However, empty non-list sequences are handled incorrectly because +since they are not the +.code nil +object, they look non-empty under this paradigm of traversal. +The +.code nullify +function provides a correction: if the input sequence is filtered +through +.code nullify +then the subsequent list-like iteration works correctly. + +Examples: + +.verb + ;; Incorrect for empty strings: + + (defun print-chars (string) + (while string + (prinl (pop string)))) + + ;; Corrected with nullify: + + (defun print-chars (string) + (let ((s (nullify string))) + (while s + (prinl (pop s))))) +.brev + +Note: optimized generic iteration is available in the form of iteration +based on +.code iter-begin +rather than +.cod3 car / cdr +and +.codn nullify . + +Examples: + +.verb + ;; Efficient with iterators, + ;; at the cost of verbosity: + + (defun print-chars (string) + (let ((i (iter-begin string))) + (while (iter-more i) + (prinl (iter-item s)) + (set s (iter-step s))))) + + ;; Using mapping function built on iterators: + + (defun print-chars (string) + [mapdo prinl string]) +.brev + .coNP Accessor @ sub .synb .mets (sub < sequence >> [ from <> [ to ]]) @@ -33181,105 +33280,6 @@ function. #(4 5 6 7)) .brev -.coNP Function @ nullify -.synb -.mets (nullify << iterable ) -.syne -.desc -The -.code nullify -function returns -.code nil -if -.meta iterable -denotes an empty sequence. -Otherwise, if -.meta iterable -is not an empty sequence, or isn't a sequence, then -.meta iterable -itself is returned. - -If -.meta iterable -is a structure object which supports the -.code nullify -method, then that method is called. If it returns -.code nil -then -.code nil -is returned. If the -.code nullify -method returns a substitute object other than the -.meta iterable -object itself, then -.code nullify -is invoked on that returned substitute object. - -Note: the -.code nullify -function is a helper to support unoptimized generic -traversal of sequences. Thanks to the generalized behavior of -.codn cdr , -non-list sequences can be traversed using -.codn cdr , -similarly to proper lists, by checking for -.code cdr -returning the terminating value -.codn nil . -However, empty non-list sequences are handled incorrectly because -since they are not the -.code nil -object, they look non-empty under this paradigm of traversal. -The -.code nullify -function provides a correction: if the input sequence is filtered -through -.code nullify -then the subsequent list-like iteration works correctly. - -Examples: - -.verb - ;; Incorrect for empty strings: - - (defun print-chars (string) - (while string - (prinl (pop string)))) - - ;; Corrected with nullify: - - (defun print-chars (string) - (let ((s (nullify string))) - (while s - (prinl (pop s))))) -.brev - -Note: optimized generic iteration is available in the form of iteration -based on -.code iter-begin -rather than -.cod3 car / cdr -and -.codn nullify . - -Examples: - -.verb - ;; Efficient with iterators, - ;; at the cost of verbosity: - - (defun print-chars (string) - (let ((i (iter-begin string))) - (while (iter-more i) - (prinl (iter-item s)) - (set s (iter-step s))))) - - ;; Using mapping function built on iterators: - - (defun print-chars (string) - [mapdo prinl string]) -.brev - .SS* Open Sequence Traversal Functions in this category perform efficient traversal of sequences. |