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(guile.info)Chaining

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Environment Chaining
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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.