diff options
Diffstat (limited to 'module/srfi')
-rw-r--r-- | module/srfi/srfi-1.scm | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/module/srfi/srfi-1.scm b/module/srfi/srfi-1.scm index 5bcc3611c..8ddf2714b 100644 --- a/module/srfi/srfi-1.scm +++ b/module/srfi/srfi-1.scm @@ -238,6 +238,10 @@ higher-order procedures." (scm-error 'wrong-type-arg caller "Wrong type argument: ~S" (list arg) '()))) +(define (out-of-range proc arg) + (scm-error 'out-of-range proc + "Value out of range: ~A" (list arg) (list arg))) + ;; the srfi spec doesn't seem to forbid inexact integers. (define (non-negative-integer? x) (and (integer? x) (>= x 0))) @@ -375,6 +379,30 @@ end-of-list checking in contexts where dotted lists are allowed." (loop (cdr prev) (cdr tail))))))) +(define (split-at lst i) + "Return two values, a list of the elements before index I in LST, and +a list of those after." + (if (< i 0) + (out-of-range 'split-at i) + (let lp ((l lst) (n i) (acc '())) + (if (<= n 0) + (values (reverse! acc) l) + (lp (cdr l) (- n 1) (cons (car l) acc)))))) + +(define (split-at! lst i) + "Linear-update variant of `split-at'." + (cond ((< i 0) + (out-of-range 'split-at! i)) + ((= i 0) + (values '() lst)) + (else + (let lp ((l lst) (n (- i 1))) + (if (<= n 0) + (let ((tmp (cdr l))) + (set-cdr! l '()) + (values lst tmp)) + (lp (cdr l) (- n 1))))))) + (define (last pair) "Return the last element of the non-empty, finite list PAIR." (car (last-pair pair))) |