Schematic: When to use let, let * and letrec?

Question

What is the difference between let, let * and letrec?

Please provide detailed explanations and examples.

Answer 1

It's best to read the official descriptions of R5RS let, let * and letrec.

Shortly speaking:

(let ((x 2)) (let ((x 3) (yx)) y) => 2 (let ((x 2)) (let* ((x 3) (yx)) y) => 3 

Thus, the difference between let and let * will evaluate all the bindings relative to the level above (therefore, it does not matter in which order they are listed), and let * do it sequentially. (let * ((xa) (by))) be equivalent (let ((xa)) (let ((by))).

Letrec, on the other hand, allows you to bind recursive values. This way you can write a recursive function that you only need within the scope and bind it to a name using letrec.