The Style

Programs are easier to read and to understand when they are written in a familiar style and follow standard coding conventions. Most organizations that develop software therefore require programmers to write programs that follow the organization’s preferred style and coding conventions. Programs are generally written once, read many times, and edited over and over again. Style conventions help both you and any other programmer using your code (such as your homework partner) to more easily understand what you were thinking when you wrote the code.

In addition to following the design recipe, all code must adhere to the following style guidelines, which are intended to make your code easier to read, debug, and edit. These guidelines will be updated throughout the semester as you learn more so please check back often to make sure you are adhering to them before you submit your homeworks.

Document Structure - General Exercises

1. Capitalize terms appropriately. Constants denote fixed values that are used in your function definitions. They should should be in ALL-CAPS, with dashes between words:

   (define MOON-RADIUS 20)

   (define SUN-IMG (circle 35 "solid" "yellow"))

   Function names, structure names, examples, and inputs should be all lower case with dashes between words (this is called kebab-case):

   (define (my-function-name my-input-name) (+ my-input-name 3))

   (define posn1 (make-posn 3 4))

   Note that posn1 is not considered a constant, since it is just the name for an example, e.g. accompanying a date definition, and not used in actual code. Hence it is not in all-caps.

   Finally, types of data should be in title case: ThisIsADataType or ThisIsAnotherOne.

2. Organize your program top-down. This means that when you write a solution that contains multiple functions, the primary function should come first, followed by helpers. The functions should be in order of where they appear in the primary function. For example, the following code is organized top-down:

   ; my-function : Number String -> Number

   ; Add double the string-length to twice the number cubed

   (check-expect (my-function 2 "hi") 20)

   (check-expect (my-function 3 "hello") 64)

   (define (my-function n s)

   (+ (double (cube-num n)) (double-length s)))

   ; double : Number -> Number

   ; Computes 2n

   (check-expect (double 4) 8)

   (define (double n) (* n 2))

   ; cube-num : Number -> Number

   ; Produces the cube of this number

   (check-expect (cube-num 5) 125)

   (define (cube-num n) (expt n 3))

   ; double-length : String -> Number

   ; Produces twice the length of this string

   (check-expect (double-length "goodbye") 14)

   (define (double-length s)

   (double (string-length s)))

   Please note that many of the above helper functions were written solely to illustrate the top-down organization we expect.

3. Title your exercises. Above every exercise please note which exercise it is. (This does not apply to project homeworks.)

4. Separate data definitions. Data definitions (and their corresponding examples/templates) should be placed at the beginning of the relevant exercise. Data definitions do not need to be repeated if used in multiple exercises.

General

1. Use names that make sense with respect to the problem, for your data definitions, field names, functions, constants, and parameters.

2. Use proper indentation. Use the indentation style of DrRacket in your program. You can go to "Racket" > "Reindent All" to indent your entire file properly. Press tab to reindent the current line, or the currently selected portion of your file.

3. Keep lines narrow. Do not exceed 102 columns for code or comments. DrRacket will show you the current line and column number in the bottom right of the window. You can also use its Edit -> Find Longest Line menu item. Or, you can go to Edit -> Preferences -> Editing -> General Editing, check the Maximum character width guide option and set it to 102.

4. Do not use dangling parentheses: the closing right-parenthesis should be on the same line as the last expression of your code.

   ;; ------------------------ GOOD

   (define (f l)

   (cond [(empty? l) 0]

   [else (f (rest l))])) ;; HERE

   ;; ------------------------ BAD

   (define (f l)

   (cond [(empty? 1) 0]

   [else (f (rest l))]

   ) ;; NOT HERE

   )

   The dangling parentheses in the second code excerpt are considered extremely bad style.

5. Break lines to break up logically distinct tasks. Consider these examples of simple function calls:

   ;; ----------------- GOOD

   (define (foo x y z)

   (max (* x y)      ;; Break after each argument to max,

   (* y z)      ;; and align all arguments in a column

   (* x z)      ;; (This works best with short-named functions)

   (* x y z)))

   ;; ----------------- OK

   (define (foo x y z)

   (max          ;; Break after max itself

   (* x y)      ;; Then indent each argument 1 space

   (* y z)      ;; (This works better when function names are long)

   (* x z)

   (* x y z)))

   ;; ----------------- BAD

   (define (foo x y z)

   (max (* x y)

   (* y z)     ;; This indentation is an inconsistent

   (* x z)     ;; mix of the previous two styles

   (* x y z)))

   ;; ----------------- BAD

   (define (foo x y z)

   (max (* x y) (* y    ;; This linebreak is just weird.

   z)

   (* x z) (* x    ;; This is ugly. And avoidable!

   y

   z)

   )

   )

   By breaking after each argument, you will more often keep complete expressions together, and need fewer line breaks overall.

   In rare cases, you can keep two arguments on a line, when they logically belong together. For example, the x- and y-coordinates in a call to place-image might easily fit on one line, and logically form a pair of coordinates, so they could stay on one line, in agreement with good style.

   Here are some more examples:

   ;; ---------------- BAD

   (define            ;; Don't break here

   (foo x y z)

   ...)

   ;; ---------------- BAD

   (define-struct      ;; Don't break here

   foo [x y z])

   (define-struct foo  ;; or here

   [x y z])

   ;; -------------------------- GOOD

   (define (foo l)

   (if (some-condition ...)

   (then-expression ...)

   (else-expression ...)))

   ;; -------------------------- BAD

   (define (foo l)

   (if (some-condition ...)

   (then-expression ...)    ;; Not aligned with condition

   (else-expression ...)))

   ;; --------------------------------- VERY GOOD

   (define (f l)                        ;; Aligning the responses

   (cond [(empty? l)  0]              ;; in a column is very legible.

   [else        (f (rest l))])) ;; ...if there's room for it

   ;; --------------------------------- GOOD

   (define (f l)

   (cond [(empty? l) 0]

   [else (f (rest l))]))

   ;; --------------------------------- GOOD

   (define (f l)

   (cond

   [(empty? l) 0]

   [else (f (rest l))]))

   ;; --------------------------------- OK

   (define (f l)

   (cond

   [(empty? l)

   0]                    ;; Only use this style if the conditions

   [else                  ;; or the responses are very long

   (f (rest l))]))       ;; (which by itself is not a good sign)

Document Structure - Large Programs

1. Organize your program top-down, regardless of how you actually work through your wish list. The phrase "top down" means that project files consist of a data definition and a constant definition section, a main function, followed by sections for handler functions, and wrapped up by general utility functions (functions used by multiple handlers). Within these sections please use top-down organization as defined above.

   The main function is the one that uses big-bang, read-file, write-file, and so on. A good purpose statement for the main function explains how to use it. For example,

   ; main : Number -> String

   ; (main n) runs the game with tick-rate n

   (define (main tick-speed)

   (... (big-bang WORLD-STATE-0

   [on-tick update-world tick-speed]

   [to-draw render-world]

   ...)))

2. Title your sections. Title your data definition section, your constant section, and each of your handler sections. Good names for your handler sections are the names of your handlers! Also label your utility functions section.

Note: the exception to this structure is when we have you work through the homeworks in a bottom-up fashion. Essentially, the "organize homeworks in order of exercise" rule takes precedence over this structure.

Data Definitions

1. Follow the design recipe. When we ask you to design data we expect you to follow all four steps of the design recipe: definition, interpretation, examples, and template.

Functions

1. Remember your signature and purpose. Each new function should come with its own corresponding signature and purpose statement. These should be written directly above the function.

2. Write proper signatures. Remember that the only types of data that you can use for a signature are pre-defined types (e.g. String, Number, etc.) or types you have defined yourself. Signatures have one or more input types followed by an arrow and one output type (see above for examples).

3. Write clear purpose statements. A purpose statement should NOT repeat the information given by the signature. It should be succinct and give the reader an understanding of what the function is doing with its inputs. It is good style for the purpose statement of a predicate (a function that outputs a Boolean) to be written as a question.

4. One task, one function. Functions should not be excessively long. If it is, split up the tasks into helper functions.

   If a function consumes a complex argument and must perform several different tasks, design several functions that all consume the argument, produce a value in the same data collection, and hand it over to the next function:

   ; update-hippo-world : HungryHungryHippoWorld -> HungryHungryHippoWorld

   (define (update-hippo-world hhhw)

   (update-time

   (move-marbles

   (eat-marbles hw))))

   ; update-time : HungryHungryHippoWorld -> HungryHungryHippoWorld

   (define (update-time hhhw) ...)

   ; move-marbles : HungryHungryHippoWorld -> HungryHungryHippoWorld

   (define (move-marbles hhhw) ...)

   ; eat-marbles : HungryHungryHippoWorld -> HungryHungryHippoWorld

   (define (eat-marbles hhhw) ...)

   Piling the code from these three functions into the first one would yield a confusing mess.

5. Test thoroughly. You should have at LEAST enough tests to cover all your code (none of your code should be highlighted in black when you run it). The exception is any function which produce side effects, such as functions that call big-bang or I/O functions like write-file.

Local Functions

Testing & Randomness

When you design functions that create (pseudo-)random results, do not give up on testing. To make this guide concrete, consider this example:

As your functions get more complicated, deploy "loops" to check entire lists, trees, forests and other complex, or randomly generated data structures:Note how one ignores its argument. Still, why would make-list in lieu of build-list not produce the correct result here?

; create-many-cupcakes : Natural -> [List-of Posn]

; create the given number of cupcakes

(define (create-many-cupcakes n)

(local ((define (one _) ; ignore argument, indicate with _

(create-cupcake SCENE-WIDTH SCENE-HEIGHT)))

(build-list n one)))

For a function like this, your property checker must use andmap to check all generated Posns:

; one? : Posn -> Boolean

; property tester

(define (one? p)

(in-range? SCENE-WIDTH SCENE-HEIGHT p))

(check-expect (andmap one? (create-many-cupcakes 100)) true)