Style Guidelines

 

 

This section discusses some style guidelines for Scheme, constructs to avoid, and pointers on good programming style and efficiency.

1. Variable naming.  In Scheme, variables must begin with a letter, and all subsequent characters may be letters, numbers, or any of the following:

      + - . * / < = > ! ? : $ % _ & ~ ^

   The length of variable names is unrestricted. Thus, one should choose descriptive variable names, avoiding abbreviations whenever practical. Customarily, predicates end with a `?', with a few exceptions, such as the numerical predicates.
2. Assignment forms. Scheme does provide an assignment  operator, set!. In addition, Scheme provides destructive versions of many functions; for instance, append!, given two lists, destructively appends the second list to the end of the first. These side-effecting  functions are called assignment forms, and their names usually end with `!'. While they are often used in real applications, from the point of view of functional programming, they should be considered forbidden.
3. cadadr and friends. Scheme provides compositions of car  and cdr  formed by inserting appropriate a's and d's between the c and the r. For instance, (cadr l1) denotes (car (cdr l1)). These compositions should be avoided, especially the longer ones. It is doubtful that anyone, even accomplished Scheme programmers, has an intuitive feel for what cadadr means, and using such forms generally means you are doing something in an ugly way.
4. Pretty-printing. Most Scheme programmers lay out their functions as I have throughout this manual, indenting the appropriate function calls and adding all the closing parentheses at the end, like so:

      (define (append l1 l2)
         (cond
            ((null? l1) l2)
            (else (cons (car l1) (append (cdr l1) l2)))))

   To simplify matching parentheses, however, you may find it convenient to line up the closing parentheses with the expression they are closing, as with the brackets in C, like so:

      (define (append l1 l2)
         (cond
            ((null? l1) l2)
            (else (cons (car l1) (append (cdr l1) l2)))
         )
      )

   This is purely a matter of personal preference.
5. Applying car and cdr to the empty list . In many implementations of Scheme, including XScheme, applying car or cdr to the empty list is valid, and returns nil in both cases. However, this behavior is not specified in the Scheme standards, and in other implementations it may cause an error or a warning. Thus, for reasons of portability and general good programming style, one should avoid situations in which (car '()) or (cdr '()) get evaluated.
6. Sequential constructs. Scheme offers a construct for sequential programming called begin . This allows sequential execution of a number of Scheme statements, with the return value of the begin being the last statement executed within its body. In addition, some constructs, such as let, perform an implicit begin. This should be avoided (except for its use in debugging, as described in Section 11), and is basically useless in the absence of assignment forms anyway.