fold with index : fold/index

でっち上げるとしたらこんな感じでしょうか。

	;; fold with index
	(define (fold/index proc init ls . lss)
	(let1 idx 0
	(apply fold (lambda args
	(begin0
	(apply proc (append args (list idx)))
	(set! idx (+ idx 1))))
	init (cons ls lss))))
	(fold/index (lambda (e acc idx)
	(acons idx e acc))
	'() '(a b c d e))
	;; ((4 . e) (3 . d) (2 . c) (1 . b) (0 . a))
	(fold/index (lambda (e1 e2 acc idx)
	(acons e1 (cons e2 idx) acc))
	'() '(a b c d e)'(f g h i j k))
	;; ((e j . 4) (d i . 3) (c h . 2) (b g . 1) (a f . 0))
	;; fold-right with index
	(use srfi-1)
	(define (fold-right/index proc init ls . lss)
	(let1 idx (- (length ls) 1)
	(apply fold-right (lambda args
	(begin0
	(apply proc (append args (list idx)))
	(set! idx (- idx 1))))
	init (cons ls lss))))

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追記

書かなくてもありました。

• vallog: fold-with-index