When we introduced compound procedures in Chapter 1, we used the substitution model of evaluation (1.1.5) to define what is meant by applying a procedure to arguments:
Once we admit assignment into our programming language, such a definition is no longer adequate. In particular, 3.1.3 argued that, in the presence of assignment, a variable can no longer be considered to be merely a name for a value. Rather, a variable must somehow designate a “place” in which values can be stored. In our new model of evaluation, these places will be maintained in structures called environments.
An environment is a sequence of frames. Each frame is a table (possibly empty) of bindings, which associate variable names with their corresponding values. (A single frame may contain at most one binding for any variable.) Each frame also has a pointer to its enclosing environment, unless, for the purposes of discussion, the frame is considered to be global. The value of a variable with respect to an environment is the value given by the binding of the variable in the first frame in the environment that contains a binding for that variable. If no frame in the sequence specifies a binding for the variable, then the variable is said to be unbound in the environment.
Figure 3.1 shows a simple environment structure consisting of three
frames, labeled I, II, and III. In the diagram, A, B, C, and D are pointers to
environments. C and D point to the same environment. The variables z
and x are bound in frame II, while y and x are bound in
frame I. The value of x in environment D is 3. The value of x
with respect to environment B is also 3. This is determined as follows: We
examine the first frame in the sequence (frame III) and do not find a binding
for x, so we proceed to the enclosing environment D and find the binding
in frame I. On the other hand, the value of x in environment A is 7,
because the first frame in the sequence (frame II) contains a binding of
x to 7. With respect to environment A, the binding of x to 7 in
frame II is said to
shadow the binding of x to 3 in frame I.
Figure 3.1: A simple environment structure.
The environment is crucial to the evaluation process, because it determines the
context in which an expression should be evaluated. Indeed, one could say that
expressions in a programming language do not, in themselves, have any meaning.
Rather, an expression acquires a meaning only with respect to some environment
in which it is evaluated. Even the interpretation of an expression as
straightforward as (+ 1 1) depends on an understanding that one is
operating in a context in which + is the symbol for addition. Thus, in
our model of evaluation we will always speak of evaluating an expression with
respect to some environment. To describe interactions with the interpreter, we
will suppose that there is a global environment, consisting of a single frame
(with no enclosing environment) that includes values for the symbols associated
with the primitive procedures. For example, the idea that + is the
symbol for addition is captured by saying that the symbol + is bound in
the global environment to the primitive addition procedure.
The overall specification of how the interpreter evaluates a combination remains the same as when we first introduced it in 1.1.3:
The environment model of evaluation replaces the substitution model in specifying what it means to apply a compound procedure to arguments.
In the environment model of evaluation, a procedure is always a pair consisting of some code and a pointer to an environment. Procedures are created in one way only: by evaluating a λ-expression. This produces a procedure whose code is obtained from the text of the λ-expression and whose environment is the environment in which the λ-expression was evaluated to produce the procedure. For example, consider the procedure definition
(define (square x) (* x x))
evaluated in the global environment. The procedure definition syntax is just syntactic sugar for an underlying implicit λ-expression. It would have been equivalent to have used
(define square (lambda (x) (* x x)))
which evaluates (lambda (x) (* x x)) and binds square to the
resulting value, all in the global environment.
Figure 3.2 shows the result of evaluating this define expression.
The procedure object is a pair whose code specifies that the procedure has one
formal parameter, namely x, and a procedure body (* x x). The
environment part of the procedure is a pointer to the global environment, since
that is the environment in which the λ-expression was evaluated to
produce the procedure. A new binding, which associates the procedure object
with the symbol square, has been added to the global frame. In general,
define creates definitions by adding bindings to frames.
Figure 3.2: Environment structure produced by evaluating (define (square x) (* x x)) in the global environment.
Now that we have seen how procedures are created, we can describe how procedures are applied. The environment model specifies: To apply a procedure to arguments, create a new environment containing a frame that binds the parameters to the values of the arguments. The enclosing environment of this frame is the environment specified by the procedure. Now, within this new environment, evaluate the procedure body.
To show how this rule is followed, Figure 3.3 illustrates the environment
structure created by evaluating the expression (square 5) in the global
environment, where square is the procedure generated in Figure 3.2.
Applying the procedure results in the creation of a new environment,
labeled E1 in the figure, that begins with a frame in which x, the
formal parameter for the procedure, is bound to the argument 5. The pointer
leading upward from this frame shows that the frame’s enclosing environment is
the global environment. The global environment is chosen here, because this is
the environment that is indicated as part of the square procedure
object. Within E1, we evaluate the body of the procedure, (* x x).
Since the value of x in E1 is 5, the result is (* 5 5), or 25.
Figure 3.3: Environment created by evaluating (square 5) in the global environment.
The environment model of procedure application can be summarized by two rules:
We also specify that defining a symbol using define creates a binding in
the current environment frame and assigns to the symbol the indicated
value.141 Finally, we
specify the behavior of set!, the operation that forced us to introduce
the environment model in the first place. Evaluating the expression
(set! ⟨variable⟩ ⟨value⟩) in some environment locates the
binding of the variable in the environment and changes that binding to indicate
the new value. That is, one finds the first frame in the environment that
contains a binding for the variable and modifies that frame. If the variable
is unbound in the environment, then set! signals an error.
These evaluation rules, though considerably more complex than the substitution model, are still reasonably straightforward. Moreover, the evaluation model, though abstract, provides a correct description of how the interpreter evaluates expressions. In Chapter 4 we shall see how this model can serve as a blueprint for implementing a working interpreter. The following sections elaborate the details of the model by analyzing some illustrative programs.
When we introduced the substitution model in 1.1.5 we showed how
the combination (f 5) evaluates to 136, given the following procedure
definitions:
(define (square x) (* x x)) (define (sum-of-squares x y) (+ (square x) (square y))) (define (f a) (sum-of-squares (+ a 1) (* a 2)))
We can analyze the same example using the environment model. Figure 3.4
shows the three procedure objects created by evaluating the definitions of
f, square, and sum-of-squares in the global environment.
Each procedure object consists of some code, together with a pointer to the
global environment.
Figure 3.4: Procedure objects in the global frame.
In Figure 3.5 we see the environment structure created by evaluating the
expression (f 5). The call to f creates a new environment E1
beginning with a frame in which a, the formal parameter of f, is
bound to the argument 5. In E1, we evaluate the body of f:
(sum-of-squares (+ a 1) (* a 2))
Figure 3.5: Environments created by evaluating (f 5) using the procedures in Figure 3.4.
To evaluate this combination, we first evaluate the subexpressions. The first
subexpression, sum-of-squares, has a value that is a procedure object.
(Notice how this value is found: We first look in the first frame of E1, which
contains no binding for sum-of-squares. Then we proceed to the
enclosing environment, i.e. the global environment, and find the binding shown
in Figure 3.4.) The other two subexpressions are evaluated by applying
the primitive operations + and * to evaluate the two combinations
(+ a 1) and (* a 2) to obtain 6 and 10, respectively.
Now we apply the procedure object sum-of-squares to the arguments 6 and
10. This results in a new environment E2 in which the formal parameters
x and y are bound to the arguments. Within E2 we evaluate the
combination (+ (square x) (square y)). This leads us to evaluate
(square x), where square is found in the global frame and
x is 6. Once again, we set up a new environment, E3, in which x
is bound to 6, and within this we evaluate the body of square, which is
(* x x). Also as part of applying sum-of-squares, we must
evaluate the subexpression (square y), where y is 10. This
second call to square creates another environment, E4, in which
x, the formal parameter of square, is bound to 10. And within E4
we must evaluate (* x x).
The important point to observe is that each call to square creates a new
environment containing a binding for x. We can see here how the
different frames serve to keep separate the different local variables all named
x. Notice that each frame created by square points to the global
environment, since this is the environment indicated by the square
procedure object.
After the subexpressions are evaluated, the results are returned. The values
generated by the two calls to square are added by sum-of-squares,
and this result is returned by f. Since our focus here is on the
environment structures, we will not dwell on how these returned values are
passed from call to call; however, this is also an important aspect of the
evaluation process, and we will return to it in detail in Chapter 5.
Exercise 3.9: In 1.2.1 we used the substitution model to analyze two procedures for computing factorials, a recursive version
(define (factorial n) (if (= n 1) 1 (* n (factorial (- n 1)))))and an iterative version
(define (factorial n) (fact-iter 1 1 n)) (define (fact-iter product counter max-count) (if (> counter max-count) product (fact-iter (* counter product) (+ counter 1) max-count)))Show the environment structures created by evaluating
(factorial 6)using each version of thefactorialprocedure.142
We can turn to the environment model to see how procedures and assignment can be used to represent objects with local state. As an example, consider the “withdrawal processor” from 3.1.1 created by calling the procedure
(define (make-withdraw balance) (lambda (amount) (if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds")))
Let us describe the evaluation of
(define W1 (make-withdraw 100))
followed by
(W1 50) 50
Figure 3.6 shows the result of defining the make-withdraw
procedure in the global environment. This produces a procedure object that
contains a pointer to the global environment. So far, this is no different
from the examples we have already seen, except that the body of the procedure
is itself a λ-expression.
Figure 3.6: Result of defining make-withdraw in the global environment.
The interesting part of the computation happens when we apply the procedure
make-withdraw to an argument:
(define W1 (make-withdraw 100))
We begin, as usual, by setting up an environment E1 in which the formal
parameter balance is bound to the argument 100. Within this
environment, we evaluate the body of make-withdraw, namely the
λ-expression. This constructs a new procedure object, whose code
is as specified by the lambda and whose environment is E1, the
environment in which the lambda was evaluated to produce the procedure.
The resulting procedure object is the value returned by the call to
make-withdraw. This is bound to W1 in the global environment,
since the define itself is being evaluated in the global environment.
Figure 3.7 shows the resulting environment structure.
Figure 3.7: Result of evaluating (define W1 (make-withdraw 100)).
Now we can analyze what happens when W1 is applied to an argument:
(W1 50) 50
We begin by constructing a frame in which amount, the formal parameter
of W1, is bound to the argument 50. The crucial point to observe is
that this frame has as its enclosing environment not the global environment,
but rather the environment E1, because this is the environment that is
specified by the W1 procedure object. Within this new environment, we
evaluate the body of the procedure:
(if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds")
The resulting environment structure is shown in Figure 3.8. The
expression being evaluated references both amount and balance.
Amount will be found in the first frame in the environment, while
balance will be found by following the enclosing-environment pointer to
E1.
Figure 3.8: Environments created by applying the procedure object W1.
When the set! is executed, the binding of balance in E1 is
changed. At the completion of the call to W1, balance is 50, and
the frame that contains balance is still pointed to by the procedure
object W1. The frame that binds amount (in which we executed the
code that changed balance) is no longer relevant, since the procedure
call that constructed it has terminated, and there are no pointers to that
frame from other parts of the environment. The next time W1 is called,
this will build a new frame that binds amount and whose enclosing
environment is E1. We see that E1 serves as the “place” that holds the local
state variable for the procedure object W1. Figure 3.9 shows the
situation after the call to W1.
Figure 3.9: Environments after the call to W1.
Observe what happens when we create a second “withdraw” object by making
another call to make-withdraw:
(define W2 (make-withdraw 100))
This produces the environment structure of Figure 3.10, which shows that
W2 is a procedure object, that is, a pair with some code and an
environment. The environment E2 for W2 was created by the call to
make-withdraw. It contains a frame with its own local binding for
balance. On the other hand, W1 and W2 have the same code:
the code specified by the λ-expression in the body of
make-withdraw.143 We see here why W1 and
W2 behave as independent objects. Calls to W1 reference the
state variable balance stored in E1, whereas calls to W2
reference the balance stored in E2. Thus, changes to the local state of
one object do not affect the other object.
Figure 3.10: Using (define W2 (make-withdraw 100)) to create a second object.
Exercise 3.10: In the
make-withdrawprocedure, the local variablebalanceis created as a parameter ofmake-withdraw. We could also create the local state variable explicitly, usinglet, as follows:(define (make-withdraw initial-amount) (let ((balance initial-amount)) (lambda (amount) (if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds"))))Recall from 1.3.2 that
letis simply syntactic sugar for a procedure call:(let ((⟨var⟩ ⟨exp⟩)) ⟨body⟩)is interpreted as an alternate syntax for
((lambda (⟨var⟩) ⟨body⟩) ⟨exp⟩)Use the environment model to analyze this alternate version of
make-withdraw, drawing figures like the ones above to illustrate the interactions(define W1 (make-withdraw 100)) (W1 50) (define W2 (make-withdraw 100))Show that the two versions of
make-withdrawcreate objects with the same behavior. How do the environment structures differ for the two versions?
Section 1.1.8 introduced the idea that procedures can have internal definitions, thus leading to a block structure as in the following procedure to compute square roots:
(define (sqrt x) (define (good-enough? guess) (< (abs (- (square guess) x)) 0.001)) (define (improve guess) (average guess (/ x guess))) (define (sqrt-iter guess) (if (good-enough? guess) guess (sqrt-iter (improve guess)))) (sqrt-iter 1.0))
Now we can use the environment model to see why these internal definitions
behave as desired. Figure 3.11 shows the point in the evaluation of the
expression (sqrt 2) where the internal procedure good-enough? has
been called for the first time with guess equal to 1.
Figure 3.11: Sqrt procedure with internal definitions.
Observe the structure of the environment. Sqrt is a symbol in the
global environment that is bound to a procedure object whose associated
environment is the global environment. When sqrt was called, a new
environment E1 was formed, subordinate to the global environment, in which the
parameter x is bound to 2. The body of sqrt was then evaluated
in E1. Since the first expression in the body of sqrt is
(define (good-enough? guess) (< (abs (- (square guess) x)) 0.001))
evaluating this expression defined the procedure good-enough? in the
environment E1. To be more precise, the symbol good-enough? was added
to the first frame of E1, bound to a procedure object whose associated
environment is E1. Similarly, improve and sqrt-iter were defined
as procedures in E1. For conciseness, Figure 3.11 shows only the
procedure object for good-enough?.
After the local procedures were defined, the expression (sqrt-iter 1.0)
was evaluated, still in environment E1. So the procedure object bound to
sqrt-iter in E1 was called with 1 as an argument. This created an
environment E2 in which guess, the parameter of sqrt-iter, is
bound to 1. Sqrt-iter in turn called good-enough? with the value
of guess (from E2) as the argument for good-enough?. This set up
another environment, E3, in which guess (the parameter of
good-enough?) is bound to 1. Although sqrt-iter and
good-enough? both have a parameter named guess, these are two
distinct local variables located in different frames. Also, E2 and E3 both
have E1 as their enclosing environment, because the sqrt-iter and
good-enough? procedures both have E1 as their environment part. One
consequence of this is that the symbol x that appears in the body of
good-enough? will reference the binding of x that appears in E1,
namely the value of x with which the original sqrt procedure was
called.
The environment model thus explains the two key properties that make local procedure definitions a useful technique for modularizing programs:
Exercise 3.11: In 3.2.3 we saw how the environment model described the behavior of procedures with local state. Now we have seen how internal definitions work. A typical message-passing procedure contains both of these aspects. Consider the bank account procedure of 3.1.1:
(define (make-account balance) (define (withdraw amount) (if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds")) (define (deposit amount) (set! balance (+ balance amount)) balance) (define (dispatch m) (cond ((eq? m 'withdraw) withdraw) ((eq? m 'deposit) deposit) (else (error "Unknown request: MAKE-ACCOUNT" m)))) dispatch)Show the environment structure generated by the sequence of interactions
(define acc (make-account 50)) ((acc 'deposit) 40) 90 ((acc 'withdraw) 60) 30Where is the local state for
acckept? Suppose we define another account(define acc2 (make-account 100))How are the local states for the two accounts kept distinct? Which parts of the environment structure are shared between
accandacc2?
140 Assignment introduces a subtlety into step 1 of the evaluation rule. As shown in Exercise 3.8, the presence of assignment allows us to write expressions that will produce different values depending on the order in which the subexpressions in a combination are evaluated. Thus, to be precise, we should specify an evaluation order in step 1 (e.g., left to right or right to left). However, this order should always be considered to be an implementation detail, and one should never write programs that depend on some particular order. For instance, a sophisticated compiler might optimize a program by varying the order in which subexpressions are evaluated.
141
If there is already a binding for the variable in the current
frame, then the binding is changed. This is convenient because it allows
redefinition of symbols; however, it also means that define can be used
to change values, and this brings up the issues of assignment without
explicitly using set!. Because of this, some people prefer
redefinitions of existing symbols to signal errors or warnings.