let: a new way to bind local vars
example
- defining a local variable
(define (circum r) (define pi 3.14) (* 2 pi r))
- can think of expression (* 2 pi r) as being parameterized by the value of pi
- so could write also
(define (circum r) ((lambda (pi) (* 2 pi r)) 3.14))
- cumbersome to write like this, so use “let” shorthand
(define (circum r) (let ((pi 3.14)) (* 2 pi r))