In the example of the previous subsection, we glossed over an important
point. The body of the let expression in that example refers not
only to the local variable s, but also to the top level variables
a, b, c and sqrt. (sqrt is the
standard Scheme procedure for calculating a square root.) If the body
of the let expression is evaluated in the context of the
local let environment, how does the evaluation get at the
values of these top level variables?
The answer is that the local environment created by a let
expression automatically has a reference to its containing environment
— in this case the top level environment — and that the Scheme
interpreter automatically looks for a variable binding in the containing
environment if it doesn’t find one in the local environment. More
generally, every environment except for the top level one has a
reference to its containing environment, and the interpreter keeps
searching back up the chain of environments — from most local to top
level — until it either finds a variable binding for the required
identifier or exhausts the chain.
This description also determines what happens when there is more than
one variable binding with the same name. Suppose, continuing the
example of the previous subsection, that there was also a pre-existing
top level variable s created by the expression:
(define s "Some beans, my lord!")
Then both the top level environment and the local let environment
would contain bindings for the name s. When evaluating code
within the let body, the interpreter looks first in the local
let environment, and so finds the binding for s created by
the let syntax. Even though this environment has a reference to
the top level environment, which also has a binding for s, the
interpreter doesn’t get as far as looking there. When evaluating code
outside the let body, the interpreter looks up variable names in
the top level environment, so the name s refers to the top level
variable.
Within the let body, the binding for s in the local
environment is said to shadow the binding for s in the top
level environment.