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Held: Wednesday, February 21, 2007
Summary: Today we consider techniques for defining local procedures, procedures that are only available to select other procedures.
let*works for recursive procedures.
letrecexpression has the format
(letrec ((name1 exp1) (name2 exp2) ... (namen expn)) body)
letrecis evaluated using the following series of steps.
nameninto the binding table. (Note that no corresponding values are entered.)
expn, giving you results
resultifor each reasonable i.
let, except that the order of entry into the binding table is changed.
letis somewhat stranger, but is handy for some problems.
lethas the format
(let name ((param1 exp1) (param2 exp2) ... (paramn expn)) body)
(letrec ((name (lambda (param1 ... paramn ) body))) (name val1 ... valn))
reverse(which I hope you recall from the exam).
(define reverse (lambda (lst) (reverse-kernel lst null))) (define reverse-kernel (lambda (remaining so-far) (if (null? remaining) so-far (reverse-kernel (cdr remaining) (cons (car remaining) so-far)))))
reverse-kernela local procedure.
(define reverse (letrec ((kernel (lambda (remaining so-far) (if (null? remaining) so-far (kernel (cdr remaining) (cons (car remaining) so-far)))))) (lambda (lst) (kernel lst null))))
create a kernel and call itis so common that the named let exists simply as a way to write that more concisely.
(define reverse (lambda (lst) (let kernel ((remaining lst) (so-far null)) (if (null? remaining) so-far (kernel (cdr remaining) (cons (car remaining) so-far))))))
I usually create these pages
on the fly, which means that I rarely
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