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Added EchoLisp
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2013-Jan-19 23:04:55.1662596; goodby
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=={{header|EchoLisp}}==
No native support. We implement an anonymous stack associated with the variable, and a few syntax rules to define the needed operations.
<lang lisp>
(define-syntax-rule (make-h-var name) (define name (stack (gensym))))
(define-syntax-rule (h-get name) (stack-top name))
(define-syntax-rule (h-set name value) (push name value))
(define-syntax-rule (h-undo name)
(begin
(pop name)
(when ( stack-empty? name) (error "no more values" 'name))
(stack-top name)))
(define-syntax-rule (h-values name) (stack->list name))
;; usage
(make-h-var x)→ x
(h-set x 42) → 42
(h-set x 666)→ 666
(h-set x 'elvis)→ elvis
(h-values x) → (42 666 elvis) ;; historized values
(h-get x) → elvis
(h-undo x)→ 666
(h-undo x) → 42
(h-undo x) → ❌ error: no more values x
</lang>
=={{header|Erlang}}==
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