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9.3.4 Variables and the VM

Consider the following Scheme code as an example:

  (define (foo a)
    (lambda (b) (list foo a b)))

Within the lambda expression, foo is a top-level variable, a is a lexically captured variable, and b is a local variable.

Another way to refer to a and b is to say that a is a “free” variable, since it is not defined within the lambda, and b is a “bound” variable. These are the terms used in the lambda calculus, a mathematical notation for describing functions. The lambda calculus is useful because it allows one to prove statements about functions. It is especially good at describing scope relations, and it is for that reason that we mention it here.

Guile allocates all variables on the stack. When a lexically enclosed procedure with free variables—a closure—is created, it copies those variables into its free variable vector. References to free variables are then redirected through the free variable vector.

If a variable is ever set!, however, it will need to be heap-allocated instead of stack-allocated, so that different closures that capture the same variable can see the same value. Also, this allows continuations to capture a reference to the variable, instead of to its value at one point in time. For these reasons, set! variables are allocated in “boxes”—actually, in variable cells. See Variables, for more information. References to set! variables are indirected through the boxes.

Thus perhaps counterintuitively, what would seem “closer to the metal”, viz set!, actually forces an extra memory allocation and indirection.

Going back to our example, b may be allocated on the stack, as it is never mutated.

a may also be allocated on the stack, as it too is never mutated. Within the enclosed lambda, its value will be copied into (and referenced from) the free variables vector.

foo is a top-level variable, because foo is not lexically bound in this example.

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