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

scheme let
Sol Bethany Reaves picture Sol Bethany Reaves · May 13, 2013 · Viewed 13.5k times · Source

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

Please give thorough explanations and examples.

Answer

Greg picture Greg · May 13, 2013

Your best bet is to read the official R5RS descriptions of let, let*, and letrec.

In short, however:

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

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

So the difference between let and let* is let will evaluate all the bindings with respect to the level above (so it doesn't matter what order they're listed in) while let* does it sequentially. (let* ((x a) (b y))) is equivalent to (let ((x a)) (let ((b y))).

letrec, on the other hand, allows you to bind recursive values. So you might write a recursive function that you only want to be within the function scope and bind it to a name using letrec.

Related questions

What's the point of lambda in scheme?

I am learning scheme. I know how to use both lambda and let expressions. However I'm struggling to figure out what the point is of using lambda. Can't you do everything with let that you can with lambda? It would …

Read More
Confused by the difference between let and let* in Scheme

Can anyone explain the difference simply? I don't think I understand the concept from the textbooks/sites I have consulted.

Read More
Scheme let statement

In scheme which is a functional programming language, there is no assignment statement. But in a let statement (let ((x 2)) (+ x 3)) You are assigning 2 to x, so why doesn't this violate the principle that there is no assignment statements in …

Read More