6. Joxa Style Guide

Copyright (C) 2012 Eric B. Merritt


This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

This work is derived from Riastradh’s Lisp Style Rules by Talor R. Cambell

This is document describes a recommended style for Joxa. Its an distilled from the best practices of the existing Lisp world and the lessons learned in Joxa itself. Its not meant to be a rigid set of rules for the style extremists. It is meant to help you get the most out of Joxa.

This guide is written primarily as a collection of guidelines, with rationale for each rule (If a guideline is missing rationale, please inform the author!). Although a casual reader might go through and read the guidelines without the rationale, such a reader would derive little value from this guide. In order to apply the guidelines meaningfully, their spirit must be understood; the letter of the guidelines serves only to hint at the spirit. The rationale is just as important as the guideline.

6.1. Standard Rules

These are the standard rules for formatting Lisp code; they are repeated here for completeness, although they are surely described elsewhere. These are the rules implemented in Emacs Lisp modes, and utilities such as Paredit.

6.1.1. Parentheses Terminology

This guide avoids the term parenthesis except in the general use of parentheses or parenthesized, because the word’s generally accepted definition, outside of the programming language, is a statement whose meaning is peripheral to the sentence in which it occurs, and not the typographical symbols used to delimit such statements.

The balanced pair of typographical symbols that mark parentheses in English text are round brackets, i.e. ( and ). There are several other balanced pairs of typographical symbols, such as square brackets (commonly called simply brackets in programming circles), i.e. [ and ]; curly braces (sometimes called simply braces), i.e. { and }; angle brackets (sometimes brokets (for broken brackets)), i.e. < and >.

In any balanced pair of typographical symbols, the symbol that begins the region delimited by the symbols is called the opening bracket or the left bracket, such as ( or`[` or { or <. The symbol that ends that region is called the right bracket or the closing bracket, such as > or } or ] or ).

6.1.2. Spacing

If any text precedes an opening bracket or follows a closing bracket, separate that text from that bracket with a space. Conversely, leave no space after an opening bracket and before following text, or after preceding text and before a closing bracket. Unacceptable

(foo(bar baz)quux)
(foo ( bar baz ) quux) Acceptable:

(foo (bar baz) quux) Rationale

This is the same spacing found in standard typography of western text. It is more aesthetically pleasing.

6.1.3. Line Separation

Absolutely do not place closing brackets on their own lines. Unacceptable

(define (factorial x)
  (if (< x 2)
      (* x (factorial (- x 1

 ) Acceptable

(define (factorial x)
  (if (< x 2)
      (* x (factorial (- x 1))))) Rationale

The parentheses grow lonely if their closing brackets are all kept separated and segregated. Exceptions to the Above Rule Concerning Line Separation

Do not heed this section unless you know what you are doing. Its title does not make the unacceptable example above acceptable.

When commenting out fragments of expressions with line comments, it may be necessary to break a line before a sequence of closing brackets

(define (foo bar)
  (list (frob bar)
        (zork bar)
        ;; (zap bar)

Finally, it is acceptable to break a line immediately after an opening bracket and immediately before a closing bracket for very long lists, especially in files under version control. This eases the maintenance of the lists and clarifies version diffs. Example

(define colour-names         ;Add more colour names to this list!

6.1.4. Parenthetical Philosophy

The actual bracket characters are simply lexical tokens to which little significance should be assigned. Lisp programmers do not examine the brackets individually, or, Azathoth forbid, count brackets; instead they view the higher-level structures expressed in the program, especially as presented by the indentation. Lisp is not about writing a sequence of serial instructions; it is about building complex structures by summing parts. The composition of complex structures from parts is the focus of Lisp programs, and it should be readily apparent from the Lisp code. Placing brackets haphazardly about the presentation is jarring to a Lisp programmer, who otherwise would not even have seen them for the most part. Indentation and Alignment

The operator of any form, i.e. the first subform following the opening round bracket, determines the rules for indenting or aligning the remaining forms. Many names in this position indicate special alignment or indentation rules; these are special operators, macros, or procedures that have certain parameter structures.

If the first subform is a non-special name, however, then if the second subform is on the same line, align the starting column of all following subforms with that of the second subform. If the second subform is on the following line, align its starting column with that of the first subform, and do the same for all remaining subforms.

In general, Emacs will indent Lisp code correctly. Run C-M-q (indent-sexp) on any code to ensure that it is indented correctly, and configure Emacs so that any non-standard forms are indented appropriately. Unacceptable

(+ (sqrt -1)
  (* x y)
  (+ p q))

   (sqrt -1)
   (* x y)
   (+ p q)) Acceptable

(+ (sqrt -1)
   (* x y)
   (+ p q))

 (sqrt -1)
 (* x y)
 (+ p q)) Rationale

The columnar alignment allows the reader to follow the operands of any operation straightforwardly, simply by scanning downward or upward to match a common column. Indentation dictates structure; confusing indentation is a burden on the reader who wishes to derive structure without matching parentheses manually. Non-Symbol Indentation and Alignment

The above rules are not exhaustive; some cases may arise with strange data in operator positions.

6.1.5. Lists

Unfortunately, style varies here from person to person and from editor to editor. Here are some examples of possible ways to indent lists whose operators are lists: Questionable

((car x)                            ;Requires hand indentation.
   (cdr x)

((car x) (cdr x)                    ;GNU Emacs
 foo) Preferable

((car x)                            ;Any Emacs
 (cdr x)
 foo) Rationale

The operands should be aligned, as if it were any other procedure call with a name in the operator position; anything other than this is confusing because it gives some operands greater visual distinction, allowing others to hide from the viewer’s sight. For example, the questionable indentation

((car x) (cdr x)

can make it hard to see that foo and (cdr x) are both operands here at the same level. However, GNU Emacs will generate that indentation by default.

6.1.6. Strings

If the form in question is meant to be simply a list of literal data, all of the subforms should be aligned to the same column, irrespective of the first subform. Unacceptable

("foo" "bar" "baz" "quux" "zot"
       "mumble" "frotz" "gargle" "mumph") Questionable, but acceptable

(3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4
   3 3 8 3 2 7 9 5 0 2 8 8 4 1 9 7 1 6 9 3 9 9 3) Acceptable

("foo" "bar" "baz" "quux" "zot"
 "mumble" "frotz" "gargle" "mumph")

  "bar" "baz" "quux" "zot"
  "mumble" "frotz" "gargle" "mumph") Rationale

Seldom is the first subform distinguished for any reason, if it is a literal; usually in this case it indicates pure data, not code. Some editors and pretty-printers, however, will indent unacceptably in the example given unless the second subform is on the next line anyway, which is why the last way to write the fragment is usually best.

6.1.7. Names

Naming is subtle and elusive. Bizarrely, it is simultaneously insignificant, because an object is independent of and unaffected by the many names by which we refer to it, and also of supreme importance, because it is what programming – and, indeed, almost everything that we humans deal with – is all about. A full discussion of the concept of name lies far outside the scope of this document, and could surely fill not even a book but a library.

Symbolic names are written with English words separated by hyphens. Scheme and Common Lisp both fold the case of names in programs; consequently, camel case is frowned upon, and not merely because it is ugly. Underscores are unacceptable separators except for names that were derived directly from a foreign language without translation. Unacceptable

append_map Acceptable


6.1.8. Funny Characters Question Marks: Predicates

Affix a question mark to the end of a name for a procedure whose purpose is to ask a question of an object and to yield a boolean answer. Such procedures are called predicates. Do not use a question mark if the procedure may return any object other than a boolean.

Examples .. code-block:: clojure

pair? procedure? proper-list?

Pronounce the question mark as if it were the isolated letter p. For example, to read the fragment (pair? object) aloud, say: pair-pee object. Exclamation Marks: Destructive Operations

Affix an exclamation mark to the end of a name for a procedure (or macro) whose primary purpose is to modify an object. This is common in lisps that support destructive operations. Joxa, of course, does not. However, this syntax is useful in situations where the intent is to modify an object.


set-car! append!

Pronounce the exclamation mark as bang. For example, to read the fragment (append! list tail) aloud, say: append-bang list tail. Asterisks: Variants, Internal Routines

Affix an asterisk to the end of a name to make a variation on a theme of the original name.


let -> let*

Prefer a meaningful name over an asterisk; the asterisk does not explain what variation on the theme the name means. with- and call-with-: Dynamic State and Cleanup

Prefix WITH- to any procedure that establishes dynamic state and calls a nullary procedure, which should be the last (required) argument. The dynamic state should be established for the extent of the nullary procedure, and should be returned to its original state after that procedure returns.



Prefix call-with- to any procedure that calls a procedure, which should be its last argument, with some arguments, and is either somehow dependent upon the dynamic state or continuation of the program, or will perform some action to clean up data after the procedure argument returns. Generally, CALL-WITH- procedures should return the values that the procedure argument returns, after performing the cleaning action.

call-with-input-file and call-with-output-file both accept a pathname and a procedure as an argument, open that pathname (for input or output, respectively), and call the procedure with one argument, a port corresponding with the file named by the given pathname. After the procedure returns, call-with-input-file and call-with-output-file close the file that they opened, and return whatever the procedure returned.

Generally, the distinction between these two classes of procedures is that call-with-... procedures should not establish fresh dynamic state and instead pass explicit arguments to their procedure arguments, whereas with-... should do the opposite and establish dynamic state while passing zero arguments to their procedure arguments.


Write heading comments with at least four semicolons; see also the section below titled ‘Outline Headings’.

Write top-level comments with three semicolons.

Write comments on a particular fragment of code before that fragment and aligned with it, using two semicolons.

Write margin comments with one semicolon.

The only comments in which omission of a space between the semicolon and the text is acceptable are margin comments.


;;;; Frob Grovel

;;; This section of code has some important implications:
;;;   1. Foo.
;;;   2. Bar.
;;;   3. Baz.

(defn (fnord zarquon)
  ;; If zob, then veeblefitz.
  (quux zot
        mumble             ;Zibblefrotz.

6.2. General Layout

Contained in the rationale for some of the following rules are references to historical limitations of terminals and printers, which are now considered aging cruft of no further relevance to today’s computers. Such references are made only to explain specific measures chosen for some of the rules, such as a limit of eighty columns per line, or sixty-six lines per page. There is a real reason for each of the rules, and this real reason is not intrinsically related to the historical measures, which are mentioned only for the sake of providing some arbitrary measure for the limit.

6.2.1. File Length

If a file exceeds five hundred twelve lines, begin to consider splitting it into multiple files. Do not write program files that exceed one thousand twenty-four lines. Write a concise but descriptive title at the top of each file, and include no content in the file that is unrelated to its title. Rationale

Files that are any larger should generally be factored into smaller parts. (One thousand twenty-four is a nicer number than one thousand.) Identifying the purpose of the file helps to break it into parts if necessary and to ensure that nothing unrelated is included accidentally.

6.2.2. Top-Level Form Length

Do not write top-level forms that exceed twenty-one lines, except for top-level forms that serve only the purpose of listing large sets of data. If a procedure exceeds this length, split it apart and give names to its parts. Avoid names formed simply by appending a number to the original procedure’s name; give meaningful names to the parts. Rationale

Top-level forms, especially procedure definitions, that exceed this length usually combine too many concepts under one name. Readers of the code are likely to more easily understand the code if it is composed of separately named parts. Simply appending a number to the original procedure’s name can help only the letter of the rule, not the spirit, however, even if the procedure was taken from a standard algorithm description. Using comments to mark the code with its corresponding place in the algorithm’s description is acceptable, but the algorithm should be split up in meaningful fragments anyway.

Rationale for the number twenty-one: Twenty-one lines, at a maximum of eighty columns per line, fits in a GNU Emacs instance running in a 24x80 terminal. Although the terminal may have twenty-four lines, three of the lines are occupied by GNU Emacs: one for the menu bar (which the author of this guide never uses, but which occupies a line nevertheless in a vanilla GNU Emacs installation), one for the mode line, and one for the minibuffer’s window. The writer of some code may not be limited to such a terminal, but the author of this style guide often finds it helpful to have at least four such terminals or Emacs windows open simultaneously, spread across a twelve-inch laptop screen, to view multiple code fragments.

6.2.3. Line Length

Do not write lines that exceed eighty columns, or if possible seventy-two. Rationale

Following multiple lines that span more columns is difficult for humans, who must remember the line of focus and scan right to left from the end of the previous line to the beginning of the next line; the more columns there are, the harder this is to do. Sticking to a fixed limit helps to improve readability.

Rationale for the numbers eighty and seventy-two: It is true that we have very wide screens these days, and we are no longer limited to eighty-column terminals; however, we ought to exploit our wide screens not by writing long lines, but by viewing multiple fragments of code in parallel, something that the author of this guide does very often. Seventy-two columns leave room for several nested layers of quotation in email messages before the code reaches eighty columns. Also, a fixed column limit yields nicer printed output, especially in conjunction with pagination; see the section ‘Pagination’ below.

6.2.4. Blank Lines

Separate each adjacent top-level form with a single blank line (i.e. two line breaks). Do not place blank lines in the middle of a procedure body, except to separate internal definitions; if there is a blank line for any other reason, split the top-level form up into multiple ones. Rationale

More than one blank line is distracting and sloppy. If the two concepts that are separated by multiple blank lines are really so distinct that such a wide separator is warranted, then they are probably better placed on separate pages anyway; see the next section, Pagination.

6.2.5. Dependencies

When writing a file or module, minimize its dependencies. If there are too many dependencies, consider breaking the module up into several parts, and writing another module that is the sum of the parts and that depends only on the parts, not their dependencies. Rationale

A fragment of a program with fewer dependencies is less of a burden on the reader’s cognition. The reader can more easily understand the fragment in isolation; humans are very good at local analyses, and terrible at global ones.

6.2.6. Naming

This section requires an elaborate philosophical discussion which the author is too ill to have the energy to write at this moment.

Compose concise but meaningful names. Do not cheat by abbreviating words or using contractions. Rationale

Abbreviating words in names does not make them shorter; it only makes them occupy less screen space. The reader still must understand the whole long name. This does not mean, however, that names should necessarily be long; they should be descriptive. Some long names are more descriptive than some short names, but there are also descriptive names that are not long and long names that are not descriptive. Here is an example of a long name that is not descriptive, from SchMUSE, a multi-user simulation environment written in MIT Scheme:


Not only is it long (sixty-four characters) and completely impenetrable, but halfway through its author decided to abbreviate some words as well!

Do not write single-letter variable names. Give local variables meaningful names composed from complete English words. Rationale

It is tempting to reason that local variables are invisible to other code, so it is OK to be messy with their names. This is faulty reasoning: although the next person to come along and use a library may not care about anything but the top-level definitions that it exports, this is not the only audience of the code. Someone will also want to read the code later on, and if it is full of impenetrably terse variable names without meaning, that someone will have a hard time reading the code.

Give names to intermediate values where their expressions do not adequately describe them. Rationale

An expression is a term that expresses some value. Although a machine needs no higher meaning for this value, and although it should be written to be sufficiently clear for a human to understand what it means, the expression might mean something more than just what it says where it is used. Consequently, it is helpful for humans to see names given to expressions.


A hash table maps foos to bars; (dict/get dict foo :false) expresses the datum that dict maps foo to, but that expression gives the reader no hint of any information concerning that datum. (let ((bar (dict/get dict foo :false))) ...) gives a helpful name for the reader to understand the code without having to find the definition of HASH-TABLE.

Index variables such as i and j, or variables such as A and D naming the car and cdr of a pair, are acceptable only if they are completely unambiguous in the scope.

Avoid functional combinators, or, worse, the point-free (or point-less) style of code that is popular in the Haskell world. At most, use function composition only where the composition of functions is the crux of the idea being expressed, rather than simply a procedure that happens to be a composition of two others. Rationale

Tempting as it may be to recognize patterns that can be structured as combinations of functional combinators – say, ‘compose this procedure with the projection of the second argument of that other one’, or (compose foo (project 2 bar)) –, the reader of the code must subsequently examine the elaborate structure that has been built up to obscure the underlying purpose. The previous fragment could have been written (fn (a b) (foo (bar b))), which is in fact shorter, and which tells the reader directly what argument is being passed on to what, and what argument is being ignored, without forcing the reader to search for the definitions of foo and bar or the call site of the final composition. The explicit fragment contains substantially more information when intermediate values are named, which is very helpful for understanding it and especially for modifying it later on.

The screen space that can be potentially saved by using functional combinators is made up for by the cognitive effort on the part of the reader. The reader should not be asked to search globally for usage sites in order to understand a local fragment. Only if the structure of the composition really is central to the point of the narrative should it be written as such. For example, in a symbolic integrator or differentiator, composition is an important concept, but in most code the structure of the composition is completely irrelevant to the real point of the code.

If a parameter is ignored, give it a meaningful name nevertheless and say that it is ignored; do not simply call it ‘ignored’.

When naming top-level bindings, assume namespace partitions unless in a context where they are certain to be absent. Do not write explicit namespace prefixes, such as foo/bar for an operation BAR in a module foo, unless the names will be used in a context known not to have any kind of namespace partitions. Rationale

Explicit namespace prefixes are ugly, and lengthen names without adding much semantic content. Joxa has its package system to separate the namespaces of names. It is better to write clear names which can be disambiguated if necessary, rather than to write names that assume some kind of disambiguation to be necessary to begin with. Furthermore, explicit namespace prefixes are inadequate to cover name clashes anyway: someone else might choose the same namespace prefix. Relegating this issue to a module system removes it from the content of the program, where it is uninteresting.


Write comments only where the code is incapable of explaining itself. Prefer self-explanatory code over explanatory comments. Avoid ‘literate programming’ like the plague. Rationale

If the code is often incapable of explaining itself, then perhaps it should be written in a more expressive language. This may mean using a different programming language altogether, or, since we are talking about Lisp, it may mean simply building a combinator language or a macro language for the purpose.

6.3. Attribution

This guide was derived from

Riastradh’s Lisp Style Rules by Taylor R. Campbell

licensed under:

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License