3.1.5 Variables and binding

Scheme allows identifiers to stand for locations containing values. These identifiers are called variables. In many cases, specifically when the location’s value is never modified after its creation, it is useful to think of the variable as standing for the value directly.

(let ((x 23)
      (y 42))
  (+ x y))
⇒ 65

In this case, the expression starting with let is a binding construct. The parenthesized structure following the let lists variables alongside expressions: the variable ‘x’ alongside ‘23’, and the variable ‘y’ alongside ‘42’. The let expression binds ‘x’ to ‘23’, and ‘y’ to ‘42’. These bindings are available in the body of the let expression, (+ x y), and only there.