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# Local Binding
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Although internal `define`s can be used for local binding, Racket
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provides three forms that give the programmer more control over
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bindings: `let`, `let*`, and `letrec`.
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## 1. Parallel Binding: `let`
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> +\[missing\] in \[missing\] also documents `let`.
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A `let` form binds a set of identifiers, each to the result of some
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expression, for use in the `let` body:
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```racket
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(let ([id expr] ...) body ...+)
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```
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The `id`s are bound “in parallel.” That is, no `id` is bound in the
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right-hand side `expr` for any `id`, but all are available in the
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`body`. The `id`s must be different from each other.
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Examples:
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```racket
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> (let ([me "Bob"])
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me)
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"Bob"
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> (let ([me "Bob"]
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[myself "Robert"]
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[I "Bobby"])
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(list me myself I))
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'("Bob" "Robert" "Bobby")
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> (let ([me "Bob"]
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[me "Robert"])
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me)
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eval:3:0: let: duplicate identifier
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at: me
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in: (let ((me "Bob") (me "Robert")) me)
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```
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The fact that an `id`’s `expr` does not see its own binding is often
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useful for wrappers that must refer back to the old value:
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```racket
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> (let ([+ (lambda (x y)
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(if (string? x)
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(string-append x y)
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(+ x y)))]) ; use original +
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(list (+ 1 2)
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(+ "see" "saw")))
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'(3 "seesaw")
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```
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Occasionally, the parallel nature of `let` bindings is convenient for
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swapping or rearranging a set of bindings:
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```racket
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> (let ([me "Tarzan"]
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[you "Jane"])
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(let ([me you]
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[you me])
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(list me you)))
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'("Jane" "Tarzan")
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```
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The characterization of `let` bindings as “parallel” is not meant to
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imply concurrent evaluation. The `expr`s are evaluated in order, even
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though the bindings are delayed until all `expr`s are evaluated.
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## 2. Sequential Binding: `let*`
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> +\[missing\] in \[missing\] also documents `let*`.
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The syntax of `let*` is the same as `let`:
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```racket
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(let* ([id expr] ...) body ...+)
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```
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The difference is that each `id` is available for use in later `expr`s,
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as well as in the `body`. Furthermore, the `id`s need not be distinct,
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and the most recent binding is the visible one.
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Examples:
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```racket
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> (let* ([x (list "Burroughs")]
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[y (cons "Rice" x)]
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[z (cons "Edgar" y)])
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(list x y z))
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'(("Burroughs") ("Rice" "Burroughs") ("Edgar" "Rice" "Burroughs"))
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> (let* ([name (list "Burroughs")]
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[name (cons "Rice" name)]
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[name (cons "Edgar" name)])
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name)
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'("Edgar" "Rice" "Burroughs")
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```
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In other words, a `let*` form is equivalent to nested `let` forms, each
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with a single binding:
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```racket
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> (let ([name (list "Burroughs")])
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(let ([name (cons "Rice" name)])
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(let ([name (cons "Edgar" name)])
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name)))
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'("Edgar" "Rice" "Burroughs")
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```
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## 3. Recursive Binding: `letrec`
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> +\[missing\] in \[missing\] also documents `letrec`.
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The syntax of `letrec` is also the same as `let`:
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```racket
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(letrec ([id expr] ...) body ...+)
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```
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While `let` makes its bindings available only in the `body`s, and `let*`
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makes its bindings available to any later binding `expr`, `letrec` makes
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its bindings available to all other `expr`s—even earlier ones. In other
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words, `letrec` bindings are recursive.
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The `expr`s in a `letrec` form are most often `lambda` forms for
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recursive and mutually recursive functions:
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```racket
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> (letrec ([swing
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(lambda (t)
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(if (eq? (car t) 'tarzan)
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(cons 'vine
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(cons 'tarzan (cddr t)))
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(cons (car t)
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(swing (cdr t)))))])
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(swing '(vine tarzan vine vine)))
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'(vine vine tarzan vine)
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```
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```racket
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> (letrec ([tarzan-near-top-of-tree?
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(lambda (name path depth)
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(or (equal? name "tarzan")
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(and (directory-exists? path)
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(tarzan-in-directory? path depth))))]
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[tarzan-in-directory?
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(lambda (dir depth)
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(cond
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[(zero? depth) #f]
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[else
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(ormap
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(λ (elem)
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(tarzan-near-top-of-tree? (path-element->string elem)
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(build-path dir elem)
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(- depth 1)))
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(directory-list dir))]))])
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(tarzan-near-top-of-tree? "tmp"
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(find-system-path 'temp-dir)
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4))
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#f
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```
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While the `expr`s of a `letrec` form are typically `lambda` expressions,
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they can be any expression. The expressions are evaluated in order, and
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after each value is obtained, it is immediately associated with its
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corresponding `id`. If an `id` is referenced before its value is ready,
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an error is raised, just as for internal definitions.
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```racket
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> (letrec ([quicksand quicksand])
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quicksand)
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quicksand: undefined;
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cannot use before initialization
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```
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## 4. Named `let`
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A named `let` is an iteration and recursion form. It uses the same
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syntactic keyword `let` as for local binding, but an identifier after
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the `let` \(instead of an immediate open parenthesis\) triggers a
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different parsing.
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```racket
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(let proc-id ([arg-id init-expr] ...)
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body ...+)
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```
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A named `let` form is equivalent to
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```racket
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(letrec ([proc-id (lambda (arg-id ...)
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body ...+)])
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(proc-id init-expr ...))
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```
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That is, a named `let` binds a function identifier that is visible only
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in the function’s body, and it implicitly calls the function with the
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values of some initial expressions.
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Examples:
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```racket
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(define (duplicate pos lst)
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(let dup ([i 0]
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[lst lst])
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(cond
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[(= i pos) (cons (car lst) lst)]
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[else (cons (car lst) (dup (+ i 1) (cdr lst)))])))
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> (duplicate 1 (list "apple" "cheese burger!" "banana"))
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'("apple" "cheese burger!" "cheese burger!" "banana")
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```
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## 5. Multiple Values: `let-values`, `let*-values`, `letrec-values`
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> +\[missing\] in \[missing\] also documents multiple-value binding forms.
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In the same way that `define-values` binds multiple results in a
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definition \(see \[missing\]\), `let-values`, `let*-values`, and
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`letrec-values` bind multiple results locally.
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```racket
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(let-values ([(id ...) expr] ...)
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body ...+)
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```
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```racket
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(let*-values ([(id ...) expr] ...)
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body ...+)
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```
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```racket
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(letrec-values ([(id ...) expr] ...)
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body ...+)
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```
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Each `expr` must produce as many values as corresponding `id`s. The
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binding rules are the same for the forms without `-values` forms: the
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`id`s of `let-values` are bound only in the `body`s, the `id`s of
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`let*-values`s are bound in `expr`s of later clauses, and the `id`s of
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`letrec-value`s are bound for all `expr`s.
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Example:
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```racket
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> (let-values ([(q r) (quotient/remainder 14 3)])
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(list q r))
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'(4 2)
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```
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