On this page:
BSL:   Vocabulary
BSL:   Grammar
BSL:   Meaning
Meaning and Computing
BSL:   Errors
Boolean Expressions
Constant Definitions
Structure Type Definitions
BSL:   Tests

Intermezzo: BSL

Fixed-Size Data deals with BSL as if it were a natural language. It introduces the “basic words” of the language, suggests how to compose the “words” into “sentences,” and appeals to your knowledge of algebra for an intuitive understanding of these “sentences.” While this kind of introduction works to some extent, truly effective communication requires some formal study of a language.

A programming language is in many ways like a natural language. It has a vocabulary and a grammar, though programmers use the word syntax for these. The vocabulary is the collection of those words from which we compose sentences in our language. A sentence in a programming language is an expression or a definition; the language’s grammar dictates how to form these phrases.

Not all grammatical sentences are meaningful—neither in English nor in a programming language. For example, the English sentence “the cat is round” is a meaningful sentence, but “the brick is a car” makes no sense even though it is completely grammatical. To determine whether a sentence is meaningful, we must know the meaning of a language; programmers call this semantics.

The intermezzos of this book introduce ideas from programming languages that programmers must eventually understand and appreciate but are secondary to program design. In this intermezzo, we discuss the meaning of BSL programs as if it were an extension of the familiar laws of arithmetic and algebra. After all, computation starts with this form of simple mathematics, and we should understand the connection between this mathematics and computing. The first three sections present the syntax and semantics of a good portion of BSL. Based on this new understanding of BSL, the fourth resumes our discussion of errors. The remaining sections expand this understanding to the complete language; the last one expands the tools for expressing tests.

BSL: Vocabulary

Figure 29 introduces and defines BSL’s basic vocabulary. It consists of literal constants, such as numbers or Boolean values; names that have meaning according to BSL, for example, cond or +; and names to which programs can give meaning via define or function parameters.

A name or a variable is a sequence of characters not including a space or one of the following: " , ' ` ( ) [ ] { } | ; #:
  • A primitive is a name to which BSL assigns meaning, for example, + or sqrt.

  • A variable is a name without preassigned meaning.

A value is one of:
  • A number is one of: 1, -1, 3/5, 1.22, #i1.22, 0+1i, and so on. The syntax for BSL numbers is complicated because it accommodates a range of formats: positive and negative numbers, fractions and decimal numbers, exact and inexact numbers, real and complex numbers, numbers in bases other than 10, and more. Understanding the precise notation for numbers requires a thorough understanding of grammars and parsing, which is out of scope for this intermezzo.

  • A boolean is one of: #true or #false.

  • A string is one of: "", "a", "doll", "he says \"hello world\" ok", and so on. In general, it is a sequence of characters enclosed by a pair of ".

  • An image is a png, jpg, tiff, and various other formats. We intentionally omit a precise definition.

We use v, v-1, v-2 and the like when we wish to say “any possible value.”

Figure 29: BSL core vocabulary

Each of the explanations defines a set via a suggestive iteration of its elements. Although it is possible to specify these collections in their entirety and in a formal manner, we consider this superfluous here and trust in your intuition. Keep in mind that each of these sets may come with some extra elements.

BSL: Grammar

Figure 30 shows a large part of the BSL grammar, which—in comparison to other languages—is extremely simple. As to BSL’s expressive power, don’t let the looks deceive you. Reading a grammar aloud makes it sound like a data definition. One could indeed use grammars to write down many of our data definitions. The first action item is to discuss how to read such grammars. Each line with a = introduces a syntactic category; the best way to pronounce = as “is one of” and | as “or.” Wherever you see three dots, imagine as many repetitions of what precedes the dots as you wish. This means, for example, that a program is either nothing or a single occurrence of def-or-expr or a sequence of two of them:
or three, four, five, or however many. Since this example is not particularly illuminating, let us look at the second syntactic category. It says that definition is either

(define (variable variable) expr)

because “as many as you wish” includes zero, or

(define (variable variable variable) expr)

which is one repetition, or

(define (variable variable variable variable) expr)

which uses two.

  program = def-or-expr ...
  def-or-expr = definition
  | expr
  definition = (define (variable variable variable ...) expr)
  expr = variable
  | value
  | (primitive expr expr ...)
  | (variable expr expr ...)
  | (cond [expr expr] ... [expr expr])
  | (cond [expr expr] ... [else expr])

Figure 30: BSL core grammar

The final point about grammars concerns the three “words” that come in a distinct font: define, cond, and else. According to the definition of BSL vocabulary, these three words are names. What the vocabulary definition does not tell us is that these names have a pre-defined meaning. In BSL, these words serve as markers that differentiate some compound sentences from others, and in acknowledgment of their role, such words are called keywords.

Now we are ready to state the purpose of a grammar. The grammar of a programming language dictates how to form sentences from the vocabulary of the grammar. Some sentences are just elements of vocabulary. For example, according to figure 30 42 is a sentence of BSL:
  • The first syntactic category says that a program is a def-or-expr.In DrRacket, a program really consists of two distinct parts: the definitions area and the expressions in the interactions area. Expressions may refer to the definitions.

  • The second definition tells us that a def-or-expr is either a definition or an expr.

  • The last definition lists all ways of forming an expr, and the second one is value.

Since we know from figure 29 that 42 is a value, we have a complete confirmation.

The interesting parts of the grammar show how to form compound sentences, which are sentences built from other sentences. For example, the definition part tells us that a function definition is formed by using “(”, followed by the keyword define, followed by another “(”, followed by a sequence of at least two variables, followed by “)”, followed by an expr, and closed by a right parenthesis “)” that matches the very first one. You can see from this example how the leading keyword define distinguishes definitions from expressions.

Expressions, called expr, come in six flavors: variables, constants, primitive applications, (function) applications, and two varieties of conditionals. While the first two are atomic sentences, the last four are compound sentences. Note how the keyword cond distinguishes conditional expressions from primitive and function applications.

Here are three examples of expressions: "all", x, and (f x). The first one belongs to the class of strings and is therefore an expression. The second is a variable, and every variable is an expression. The third is a function application, because f and x are variables.

In contrast, the following parenthesized sentences are not expressions: (f define), (cond x), and ((f 2) 10). The first one partially matches the shape of a function application but it uses define as if it were a variable. The second one fails to be a correct cond expression because it contains a variable as the second item and not a pair of expressions surrounded by parentheses. The last one is neither a conditional nor an application because the first part is an expression.

Finally, you may notice that the grammar does not mention white space: blank spaces, tabs, and newlines.Keep in mind that two kinds of readers study your BSL programs: people and DrRacket. BSL is a permissive language. As long as there is some white space between the elements of any sequence in a program, DrRacket can understand your BSL programs. Good programmers, however, may not like what you write. These programmers use white space to make their programs easily readable. Most importantly, they adopt a style that favors human readers over the software applications that process programs (such as DrRacket). They pick up this style from careful reading code examples in books, paying attention to how it is formatted.

Exercise 117. Explain why the following sentences are syntactically legal expressions
  1. x

  2. (= y z)

  3. (= (= y z) 0)


Exercise 118. Explain why the following sentences are syntactically illegal
  1. (3 + 4)

  2. number?

  3. (l)

  4. (x)


Exercise 119. Explain why the following sentences are syntactically legal definitions
  1. (define (f x) x)

  2. (define (f x) y)

  3. (define (f x y) 3)


Exercise 120. Explain why the following sentences are syntactically illegal
  1. (define (f "x") x)

  2. (define (f x y z) (x))

  3. (define (f) 10)


Exercise 121. Discriminate the legal from the illegal sentences in the following list:
  1. (x)

  2. (+ 1 (not x))

  3. (+ 1 2 3)

Explain why the sentences are legal or illegal. Determine whether they belong the category expr or definition. image

Note on Grammatical Terminology The components of compound sentences have names. We have introduced some of these names on an informal basis. Figure 31 provides a summary of the conventions.

; function application:
(function argument ... argument)
; function definition:
(define (function-name parameter ... parameter)
; conditional expression:
; cond clause
[condition answer]

Figure 31: Syntactic naming conventions

In addition to the terminology of figure 31, we say function header for second component of a definition. Accordingly, the expression component is called function body. People who consider programming languages as a form of mathematics use left-hand side for a header and right-hand side for the body. On occasion, you also hear or read the term actual arguments for the arguments in a function application.

BSL: Meaning

When you hit the return key on your keyboard and ask DrRacket to evaluate an expression, it uses the laws of arithmetic and algebra to obtain a value. For the variant of BSL treated so far, figure 29 defines grammatically what a value is—the set of values is just a subset of all expressions. The set includes Booleans, String, and Images.

The rules of evaluation come in two categories. First, we need an infinite number of rules like those of arithmetic to evaluate applications of primitives:
(+ 1 1) == 2
(- 2 1) == 1
But BSL arithmetic is more general than just number crunching. It also includes rules for dealing with Boolean values, strings, and so on:
(not #true) == #false
(string=? "a" "a") == #true
Remember == says that two expressions are equal according to the laws of computation in BSL.

Like in algebra, you can always replace equals with equals in expressions:
(boolean? (= (string-length (string-append "hello" "world")) (+ 1 3)))
(boolean? (= (string-length (string-append "hello" "world")) 4))
(boolean? (= (string-length "helloworld") 4))
(boolean? (= 10 4))
(boolean? #false)

Second, we need a rule from algebra to understand the application of a functions to arguments. Suppose the program contains the definition
(define (f x-1 ... x-n)
Then an application of a function is governed by the law:
(f v-1 ... v-n) == f-body
; with all occurrences of x-1 ... x-n
; replaced with v-1 ... v-n, respectively
Due to the history of languages such as BSL, we refer to this rule as the beta or beta-value rule.

This rule is as general as possible, so it is best to look at a concrete example. Say the definition is
(define (poly x y)
  (+ (expt 2 x) y))
Then the first step in an evaluation of (poly 3 5) uses beta:

(poly 3 5) == (+ (expt 2 3) 5) ... == (+ 8 5) == 13

For the rest, DrRacket uses plain arithmetic.

In addition to beta, we need a couple of rules that help us determine the value of cond expressions. These rules are algebraic rules but are not a part of the standard algebra curriculum. When the first condition is #false, then the first cond-line disappears:
  [#false ...]
  [condition-1 answer-1]




  ; the first line disappeared
  [condition-1 answer-1]
This rule has the name condfalse. When the first condition is #true, DrRacket picks the right-hand side of this cond clause:
  [#true answer]
  [condition-1 answer-1]




We call this rule condtrue. It also applies when the condition is else.

Consider the following evaluation:
  [(zero? 3) 1]
  [(= 3 3) (+ 1 1)]
  [else 3])
== ; by plain arithmetic and equals for equals
  [#false 1]
  [(= 3 3) (+ 1 1)]
  [else 3])
== ; by rule condfalse
  [(= 3 3) (+ 1 1)]
  [else 3])
== ; by plain arithmetic and equals for equals
  [#true (+ 1 1)]
  [else 3])
== ; by rule condtrue
(+ 1 1)
The calculation illustrates plain arithmetic, the replacement of equals by equals, and the two cond rules.

Exercise 122. Evaluate the following expressions step by step:
  1. (+ (* (/ 12 8) 2/3)
       (- 20 (sqrt 4)))
  2. (cond
      [(= 0 0) #false]
      [(> 0 1) (string=? "a" "a")]
      [else (= (/  1 0) 9)])
  3. (cond
      [(= 2 0) #false]
      [(> 2 1) (string=? "a" "a")]
      [else (= (/  1 2) 9)])
Use DrRacket’s stepper to confirm your computations. image

Exercise 123. Suppose the program contains these definitions:
(define (f x y)
  (+ (* 3 x) (* y y)))
Show how DrRacket evaluates the following expressions, step by step:
  1. (+ (f 1 2) (f 2 1))

  2. (f 1 (* 2 3))

  3. (f (f 1 (* 2 3)) 19)

Use DrRacket’s stepper to confirm your computations. image

Meaning and Computing

Scientists would say the stepper is a model of DrRacket’s actual evaluation mechanism. The interpreters from Refining Interpreters is another model.

The stepper tool in DrRacket mimics a student in a pre-algebra course. In a sense, the stepper is like a mechanized student from a pre-algebra course, and it is extremely good at, and also extremely fasy, applying the laws of arithmetic and algebra as spelled out here.

You can, and you ought to, use the stepper when you don’t understand how a new language construct works. The sections on Computing suggest exercises for this purpose, but you can make up your own examples, run them through the stepper, and ponder why it takes certain steps.

Finally, you may also wish to use the stepper when you are surprised by the result that a program computes. Using the stepper effectively in this way requires practice. For example, it often means copying the program and pruning unnecessary pieces. But once you understand how to use the stepper well this way, you will find that this procedure clearly explains run-time errors and logical mistakes in your programs.

BSL: Errors

When DrRacket discovers that some parenthesized phrase does not belong to BSL, it signals a syntax error. To determine whether a fully-parenthesized program is syntactically legal, DrRacket uses the grammar in figure 30 and reasons along the lines explained above. Not all syntactically legal programs have meaning, however.

When DrRacket evaluates a syntactically legal program and discovers that some operation is used on the wrong kind of value, it raises a run-time error. Consider the simple example of (/ 1 0). It is a syntactically legal sentence, but you know from mathematics that 1/0 does not have a value. Since BSL’s calculations must be consistent with mathematics, DrRacket signals an error:
> (/ 1 0)

/: division by zero

Naturally it also signals an error when an expression such as (/ 1 0) is nested deeply inside of an another expression:
> (+ (* 20 2) (/ 1 (- 10 10)))

/: division by zero

DrRacket’s behavior translates into our calculations as follows. When we find an expression that is not a value and when the evaluation rules allow no further simplification, we say the computation is stuck. This notion of stuck corresponds to a run-time error. For example, computing the value of the above expression leads to a stuck state:
(+ (* 20 2) (/ 1 (- 10 10)))
(+ (* 20 2) (/ 1 0))
(+ 40 (/ 1 0))
What this calculation also shows is that DrRacket eliminates the context of a stuck expression as it signals an error. In this concrete example, it eliminates the addition of 40 to the stuck expression (/ 1 0).

Not all nested stuck expressions end up signaling errors. Suppose a program contains this definition:
(define (my-divide n)
    [(= n 0) "inf"]
    [else (/ 1 n)]))
If you now apply my-divide to 0, DrRacket calculates as follows:
(my-divide 0)
  [(= 0 0) "inf"]
  [else (/ 1 0)])
It would obviously be wrong to say that the function signals the division-by-zero error now, even though an evaluation of the shaded sub-expression may suggest it. The reason is that (= 0 0) evaluates to #true and therefore the second cond clause does not play any role:
(my-divide 0)
  [(= 0 0) "inf"]
  [else (/ 1 0)])
  [#true "inf"]
  [else (/ 1 0)])

Fortunately, our laws of evaluation take care of these situations automatically. We just need to keep in mind when the laws apply. For example, in

(+ (* 20 2) (/ 20 2))

the addition cannot take place before the multiplication or division. Similarly, the shaded division in
  [(= 0 0) "inf"]
  [else (/ 1 0)])
cannot be substituted for the complete cond expression until the corresponding line is the first condition in the cond.

As a rule of thumb, it is best to keep the following in mind:

Simplify the outermost and left-most sub-expression that is ready for evaluation.

While this guideline is a simplification, it always explains BSL’s results.

In some cases, programmers also want to define functions that raise errors. Recall the checked version of area-of-disk from Input Errors:
(define (checked-area-of-disk v)
    [(number? v) (area-of-disk v)]
    [else (error "number expected")]))
Now imagine applying checked-area-of-disk to a string:
(- (checked-area-of-disk "a")
   (checked-area-of-disk 10))
(- (cond
     [(number? "a") (area-of-disk "a")]
     [else (error "number expected")])
   (checked-area-of-disk 10))
(- (cond
     [#false (area-of-disk "a")]
     [else (error "number expected")])
    (checked-area-of-disk 10))
(- (error "number expected")
   (checked-area-of-disk 10))
At this point you might try to evaluate the second expression, but even if you do find out that its result is roughly 314, your calculation must eventually deal with the error expression, which is just like a stuck expression. In short, the calculation ends in
(error "number expected")

Boolean Expressions

Our current definition of BSL omits two forms of expressions: and and or expressions. Adding them provides a case study of how to study new language constructs. We must first understand their syntax and then their semantics.

Here is the revised grammar of expressions:
  expr = ...
  | (and expr expr)
  | (or expr expr)
The grammar says that and and or are keywords, each followed by two expressions. At first glance, the two expressions look like function applications.

To understand why and and or are not BSL-defined functions, we must look at their pragmatics first. Suppose we need to formulate a condition that determines whether (/ 1 n) is r:
(define (check n r)
  (and (not (= n 0)) (= (/ 1 n) r)))
We formulate the condition as an and combination of two Boolean expressions because we don’t wish to divide by 0 accidentally. Now let us apply check to 0 and 1/5:
(check 0 1/5)
(and (not (= 0 0)) (= (/ 1 0) 1/5))
If and were an ordinary operation, we would have to evaluate both sub-expressions, and doing so would trigger an error. Instead and is not a primitive operation, and its evaluation throws away the second expression when the first one is #false. In short an and expression short-circuits evaluation.

Now it would be easy to formulate evaluation rules for and and or. Another way to explain their meaning is to show how to translate them into equivalent expressions:
(and exp-1 exp-2)


is short for


  [exp-1 (if exp-2 #true #false)]
  [else #false])
(or exp-1 exp-2)


is short for


  [exp-1 #true]
  [else (if exp-2 #true #false)])
So if you are ever in doubt about how to evaluate an and or or expressions, use the above equivalences to calculate. But we trust that you understand these operations intuitively, and that is almost always enough.

Note You may be wondering why the above abbreviations use

(if exp-2 #true #false)

on the right-hand side instead of just


And indeed, if exp-2 evaluates to #true or #false, writing exp-2 suffices and is shorter than the if expression. An expression in BSL may evaluate to all kinds of values, however. If it does evaluate to something other than a Boolean, (if exp-2 #true #false) would signal an error while exp-2 would not. Since (and exp-1 exp-2) also signals an error when exp-2 evaluates to something other than a Boolean, we use the above right-hand side and not the inaccurate short-cut.

Exercise 124. The use of if may have surprised you in another way because this intermezzo does not mention this form elsewhere. In short, the intermezzo appears to explain and with a form that has no explanation either. At this point, we are relying on your intuitive understanding of if as a short-hand for cond. Write down a rule that shows how to reformulate

(if exp-test exp-then exp-else)

as a cond expression. image

Constant Definitions

Programs consist not only of function definitions but also constant definitions, but these weren’t included in our first grammar. So here is an extended grammar that includes constant definitions:
  definition = ...
  | (define name expr)
The shape of a constant definition is similar to that of a function definition. It starts with a “(”, followed by the keyword define, followed by a variable, followed by an expression, and closed by a right parenthesis “)” that matches the very first one. While the keyword define distinguishes constant definitions from expressions, it does not differentiate from function definitions. For that, a human reader must look at the second component of the definition. As it turns out DrRacket has another way of dealing with function definitions; see Nameless Functions.

Next we must understand what a constant definition means. For a constant definition with a literal constant on the right hand side, such as

(define RADIUS 5)

it is easy to see that the variable is just a short hand for the value. That is, wherever DrRacket encounters RADIUS during an evaluation, it replaces it with 5.

For a constant definition with a proper expression on the right-hand side, say,

(define DIAMETER (* 2 RADIUS))

we must immediately determine the value of the expression. This process makes use whatever definitions precede this constant definition. Hence,
(define RADIUS 5)
(define DIAMETER (* 2 RADIUS))
is equivalent to
(define RADIUS 5)
(define DIAMETER 10)

This recipe even works when function definitions are involved:
(define RADIUS 10)
(define DIAMETER (* 2 RADIUS))
(define (area r) (* 3.14 (* r r)))
(define AREA-OF-RADIUS (area RADIUS))
As DrRacket steps through this sequence of definitions, it first determines that RADIUS stands for 10, DIAMETER for 20, and area is the name of a function. Finally, it evaluates (area RADIUS) to 314 and associates AREA-OF-RADIUS with that value.

Mixing constant and function definitions gives rise to a new kind of run-time error, too. Take a look at this program:
(define RADIUS 10)
(define DIAMETER (* 2 RADIUS))
(define AREA-OF-RADIUS (area RADIUS))
(define (area r) (* 3.14 (* r r)))
It is like the one above with the last two definitions swapped. For the first two definitions, evaluation proceeds as before. For the third one, however, evaluation goes wrong. The recipe calls for the evaluation of (area RADIUS). While the definition of RADIUS precedes this expression, the definition of area has not yet been encountered. If you were to evaluate the program with DrRacket, you would therefore get an error, explaining that ``this function is not defined.'' So be carefully about using functions in constant definitions only when you know they are defined.

Exercise 125. Evaluate the following program, step by step:
(define PRICE 5)
(define SALES-TAX (* 0.08 PRICE))
Does the evaluation of the following program signal an error?
(define COLD-F 32)
(define COLD-C (fahrenheit->celsius COLD-F))
(define (fahrenheit->celsius f)
 (* 5/9 (- f 32)))
How about the next one?
(define LEFT -100)
(define RIGHT 100)
(define (f x) (+ (* 5 (expt x 2)) 10))
(define f@LEFT (f LEFT))
(define f@RIGHT (f RIGHT))
Check your computations with DrRacket’s stepper. image

Structure Type Definitions

As you can imagine, define-struct is by far the most complex BSL construct. We have therefore saved its explanation for last. Here is the addition to the grammar:
  definition = ...
  | (define-struct name [name ...])
A structure type definition is a third form of definition. The keyword define-struct distinguishes this form of definition from function and constant definitions.

Here is a simple example:

(define-struct point [x y z])

Since point, x, y, and z are variables and the parentheses are placed according to the grammatical pattern, it is a proper definition of a structure type. In contrast, these two parenthesized sentences
(define-struct [point x y z])
(define-struct point x y z)
are illegal definitions, because define-struct is not followed by a single variable name and a sequence of variables in parentheses.

While the syntax of define-struct is straightforward, its meaning is difficult to spell out with evaluation rules. As mentioned several times, a define-struct definition defines several functions at once: a constructor, several selectors, and a predicate. Thus the evaluation of

(define-struct c [s-1 ... s-n])

introduces into the following functions into the program:
  1. make-c: a constructor;

  2. c-s-1... c-s-n: a series of selectors; and

  3. c?: a predicate.

These functions have the same status as +, -, or *. Before we can understand the rules that govern these new functions, however, we must return to the definition of values. After all, one purpose of a define-struct is to introduce a class of values that is distinct from all existing values.

Simply put, the use of define-struct extends the universe of values. To start with, it now also contains structures, which compound several values into one. When a program contains a define-struct definition, its evaluation modifies the definition of values:

A value is one of: a number, a boolean, a string, an image,
  • or a structure value:
    (make-c _value-1 ... _value-n)
    assuming a structure type c is defined.

For example, the definition of point means the addition of values of this shape:
(make-point 1 2 -1)
(make-point "one" "hello" "world")
(make-point 1 "one" (make-point 1 2 -1))

Now we are in a position to understand the evaluation rules of the new functions. If c-s-1 is applied to a c structure, it returns the first component of the value. Similarly, the second selector extracts the second component, the third selector the third component, and so on. The relationship between the new data constructor and the selectors is best characterized with n equations added to BSL’s rules:

(c-s-1 (make-c V-1 ... V-n))





(c-s-n (make-c V-1 ... V-n))





For our running example, we get the specific equations

(point-x (make-point V U W))





(point-y (make-point V U W))





(point-z (make-point V U W))





When DrRacket encounters (point-y (make-point 3 4 5)) in a calculation, it equates it with 4 and (point-x (make-point (make-point 1 2 3) 4 5)) to (make-point 1 2 3).

The predicate c? can be applied to any value. It returns #true if the value is of kind c and #false otherwise. We can translate both parts into two equations:

(c? (make-c V-1 ... V-n))





(c? V)





if V is a value not constructed with make-c. Again, the equations are best understood in terms of our example:

(point? (make-point U V W))





(point? X)





if X is a value but not a point structure. Thus, (point? (make-point 3 4 5)) is #true and (point? 3) is #false.

Exercise 126. Pick the legal from the legal from illegal sentences in the following list:
  1. (define-struct oops [])

  2. (define-struct child [parents dob date])

  3. (define-struct (child person) [dob date])

Explain why the sentences are legal or illegal. image

Exercise 127. Identify which of the following expressions are values:
  1. (make-point 1 2 3)

  2. (make-point (make-point 1 2 3) 4 5)

  3. (make-point (+ 1 2) 3 4)

Explain why the expressions are values or not. image

Exercise 128. Suppose the program contains

(define-struct ball [x y speed-x speed-y])

Predict the results of evaluating the following expression:
  1. (number? (make-ball 1 2 3 4))

  2. (ball-speed-y (make-ball (+ 1 2) (+ 3 3) 2 3))

  3. (ball-y (make-ball (+ 1 2) (+ 3 3) 2 3))

  4. (ball-x (make-posn 1 2))

  5. (ball-speed-y 5)

Check your predictions in DrRacket’s interactions area. Use the stepper to see how DrRacket produces this result. image

BSL: Tests

Figure 32 collects all the elements of BSL that this intermezzo explains plus several testing forms: check-expect, check-within, check-member-of, check-range, check-error, check-random, and check-satisfied. The actual grammar of BSL is even a bit larger; see the Help Desk for details.

  program = def-or-expr ...
  def-or-expr = definition
  | expr
  | test-case
  definition = (define (name variable variable ...) expr)
  | (define name expr)
  | (define-struct name [name ...])
  expr = (name expr expr ...)
  | (cond [expr expr] ... [expr expr])
  | (cond [expr expr] ... [else expr])
  | (and expr expr expr ...)
  | (or expr expr expr ...)
  | name
  | number
  | string
  | image
  test-case = (check-expect expr expr)
  | (check-within expr expr expr)
  | (check-member-of expr expr ...)
  | (check-range expr expr expr)
  | (check-error expr)
  | (check-random expr expr)
  | (check-satisfied expr name)

Figure 32: BSL, Full Grammar

The general meaning of testing expressions is easy to explain. When you click the button{run} button, DrRacket collects all testing expressions and moves them from the middle of the program to the end, retaining the order in which they appear. It then evaluates the definitions, expressions, and tests and gets the interactions area ready.

Each test evaluates its pieces and then compares them with the expected outcome via some predicate. Beyond that, tests communicate with DrRacket to collect some success and failure statistics and information on how to display test failures.

For details, read the documentation on these test forms. Here are some illustrative examples:
; check-expect compares the outcome and the expected value with equal?
(check-expect 3 3)
; check-member-of compares the outcome and the expected values with equal?
; if one of them yields #true, the test succeeds
(check-member-of "green" "red" "yellow" "green")
; check-within compares the outcome and the expected value with a predicate
;  like equal? but allows for a tolerance of epsilon for each inexact number
(check-within (make-posn #i1.0 #i1.1) (make-posn #i0.9 #i1.2) 0.2)
; check-range is like check-within
; but allows for a specification of an interval
(check-range 0.9 #i0.6 #i1.0)
; check-error checks whether an expression signals (any) error
(check-error (/ 1 0))
; check-random evaluates the sequences of calls to random in the
; two expressions such that they yield the same number
(check-random (make-posn (random 3) (random 9))
              (make-posn (random 3) (random 9)))
; check-satisfied determines whether a predicate produces #true
; when applied to the outcome, that is, whether outcome has a certain property
(check-satisfied 4 even?)
All of the above tests succeed. The remainig parts of the book re-introduce these test forms as needed.

Exercise 129. Copy and paste the following tests into DrRacket’s definitions area:
(check-expect 3 4)
(check-member-of "green" "red" "yellow" "grey")
(check-within (make-posn #i1.0 #i1.1) (make-posn #i0.9 #i1.2) 0.01)
(check-range #i0.9 #i0.6 #i0.8)
(check-error (/ 1 1))
(check-random (make-posn (random 3) (random 9))
              (make-posn (random 9) (random 3)))
(check-satisfied 4 odd?)
Validate that all of them fail and explain why. image