Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.
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This example uses closure to create two procedures, get-balance
and deposit
, that both refer to the same captured local
environment so that they can both access the balance
variable
binding inside that environment. The value of this variable binding
persists between calls to either procedure.
Note that the captured balance
variable binding is private to
these two procedures: it is not directly accessible to any other code.
It can only be accessed indirectly via get-balance
or
deposit
, as illustrated by the withdraw
procedure.
(define get-balance #f) (define deposit #f) (let ((balance 0)) (set! get-balance (lambda () balance)) (set! deposit (lambda (amount) (set! balance (+ balance amount)) balance))) (define (withdraw amount) (deposit (- amount))) (get-balance) ⇒ 0 (deposit 50) ⇒ 50 (withdraw 75) ⇒ -25
An important detail here is that the get-balance
and
deposit
variables must be set up by define
ing them at top
level and then set!
ing their values inside the let
body.
Using define
within the let
body would not work: this
would create variable bindings within the local let
environment
that would not be accessible at top level.