Question

let-forms are allowed to contain several expressions inside:

(let ((x 4))
   x
  (+ x 1))

returns 5.

How is this expression evaluated?

Answer 1

The situation you’re describing occurs in several parts in Scheme, not only in a let expression. In the following …

• After the list of variable bindings in a let expression
• After the list of parameters in a lambda expression (and consequently, after the list of parameters in a procedure definition)
• After each clause in a cond expression

… you can write a list of expressions. Implicitly those expressions are enclosed inside a begin special form, the order of evaluation is from left to right, all the expressions are evaluated in turn but the value returned is that of the last expression.

For example, this expression:

(cond ((= 1 1) 1 2 3))

Is equivalent to:

(cond ((= 1 1) (begin 1 2 3)))

In both cases, the returned value is 3, because that’s the value of the last expression in the list.