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Google Common Lisp Style Guide updated to 1.13.

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@ -5,7 +5,7 @@
<p align="right"> <p align="right">
Revision 1.12 Revision 1.13
</p> </p>
@ -46,8 +46,27 @@ Robert Brown
<p> <p>
Common Lisp is a powerful multiparadigm programming language. Common Lisp is a powerful multiparadigm programming language.
With great power comes great responsibility. With great power comes great responsibility.
</p>
<p>
This guide recommends formatting and stylistic choices This guide recommends formatting and stylistic choices
designed to make your code easier for other people to understand. designed to make your code easier for other people to understand.
Its guidelines apply to all the Common Lisp code we write at Google,
including our internal applications as well as
free software libraries we maintain and publish.
If you contribute to this code base,
you must follow these guidelines before you submit your patch,
or your code will be fixed to follow the guidelines after you do.
</p>
<p>
If you're writing Common Lisp code outside Google,
we invite you to consider the guidelines we expound in this guide.
You may apply some or all of them to your own projects, at your leisure.
If you do, we hope that you will find some of them useful.
Even if you don't, we also welcome any remarks or constructive feedback
you may have on how to improve our guide, and
about what alternate styles are available.
Finally, we hope that our discussions will be thought-inspiring
to hackers who read them, especially so to newcomers to Common Lisp.
</p> </p>
<p> <p>
@ -141,8 +160,9 @@ Robert Brown
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p> <p>
Permission comes from the OWNERS of your project. Permission comes from the owners of your project.
</p> </p>
<p> <p>
Forgiveness is requested in a comment Forgiveness is requested in a comment
near the point of guideline violation, near the point of guideline violation,
@ -168,7 +188,16 @@ Robert Brown
Whatever the case may be, you must still follow these guidelines, Whatever the case may be, you must still follow these guidelines,
as well as other conventional guidelines as well as other conventional guidelines
that have not been formalized in this document. that have not been formalized in this document.
You MUST follow conventions, because they are important for readability. </p>
<p>
You MUST follow conventions.
They are important for readability.
When conventions are followed by default,
violations of the convention are a signal
that something notable is happening and deserves attention.
When conventions are systematically violated,
violations of the convention are a distracting noise
that needs to be ignored.
</p> </p>
<p> <p>
Conventional guidelines <em>are</em> indoctrination. Conventional guidelines <em>are</em> indoctrination.
@ -451,7 +480,7 @@ Robert Brown
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p> <p>
Use your judgement to distinguish Use your judgment to distinguish
general-purpose versus business-specific code, general-purpose versus business-specific code,
and open-source the general-purpose parts, and open-source the general-purpose parts,
while keeping the business-specific parts a trade secret. while keeping the business-specific parts a trade secret.
@ -630,8 +659,10 @@ Robert Brown
<BODY> <BODY>
<p> <p>
Some line length restriction is better than none at all. Some line length restriction is better than none at all.
Allowing 100 columns seems better, since good style encourages While old text terminals used to make 80 columns the standard,
the use of descriptive variables and function names. these days, allowing 100 columns seems better,
since good style encourages the use of
descriptive variables and function names.
</p> </p>
</BODY> </BODY>
@ -890,25 +921,31 @@ Robert Brown
<CATEGORY title="Documentation"> <CATEGORY title="Documentation">
<STYLEPOINT title="Document everything"> <STYLEPOINT title="Document everything">
<SUMMARY> <SUMMARY>
You should use document strings to explain how to use your code. You should use document strings on all visible functions
to explain how to use your code.
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p> <p>
You must comment anything complicated Unless some bit of code is painfully self-explanatory,
so that the next developer can understand what's going on. document it with a documentation string (also known as docstring).
(Again, the "hit by a truck" principle.)
</p> </p>
<p> <p>
Unless some bit of code is painfully self-explanatory, document it. Documentation strings are destined to be read
Prefer documentation strings to comments by the programmers who use your code.
because the former can be displayed by programming tools, such as IDEs, They can be extracted from functions, types, classes, variables and macros,
and displayed by programming tools, such as IDEs,
or by REPL queries such as <code>(describe 'foo)</code>; or by REPL queries such as <code>(describe 'foo)</code>;
they can also be extracted web-based documentation or other reference works
to create web-based documentation or other reference works. can be created based on them.
Documentation strings are thus the perfect locus to document your API.
They should describe how to use the code
(including what pitfalls to avoid),
as opposed to how the code works (and where more work is needed),
which is what you'll put in comments.
</p> </p>
<p> <p>
Supply a documentation string (also known as docstring) when defining Supply a documentation string when defining
top-level functions, types, classes, and macros. top-level functions, types, classes, variables and macros.
Generally, add a documentation string wherever the language allows. Generally, add a documentation string wherever the language allows.
</p> </p>
<p> <p>
@ -934,9 +971,7 @@ Robert Brown
nil) nil)
(t (t
(loop for i from 3 upto (sqrt n) by 2 (loop for i from 3 upto (sqrt n) by 2
do (when (divisorp i n) never (divisorp i n)))))
(return-from small-prime-number-p nil)))
t)))
</CODE_SNIPPET> </CODE_SNIPPET>
<CODE_SNIPPET> <CODE_SNIPPET>
(defgeneric table-clear (table) (defgeneric table-clear (table)
@ -982,6 +1017,28 @@ Robert Brown
You must use the appropriate number of semicolons to introduce comments. You must use the appropriate number of semicolons to introduce comments.
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p>
Comments are explanations to the future maintainers of the code.
Even if you're the only person who will ever see and touch the code,
even if you're either immortal and never going to quit,
or unconcerned with what happens after you leave
(and have your code self-destruct in such an eventuality),
you may find it useful to comment your code.
Indeed, by the time you revisit your code,
weeks, months or years later,
you will find yourself a different person from the one who wrote it,
and you will be grateful to that previous self
for making the code readable.
</p>
<p>
You must comment anything complicated
so that the next developer can understand what's going on.
(Again, the "hit by a truck" principle.)
</p>
<p>
Also use comments as a way to guide those who read the code,
so they know what to find where.
</p>
<ul> <ul>
<li> <li>
File headers and important comments File headers and important comments
@ -1031,14 +1088,12 @@ Robert Brown
nil) nil)
((= n 2) ; parenthetical remark here ((= n 2) ; parenthetical remark here
t) ; continuation of the remark t) ; continuation of the remark
((divisible-by n 2) ((divisorp 2 n)
nil) ; different remark nil) ; different remark
;; Comment that applies to a section of code. ;; Comment that applies to a section of code.
(t (t
(loop for i from 3 upto (sqrt n) by 2 (loop for i from 3 upto (sqrt n) by 2
do (when (divisorp i n) never (divisorp i n)))))
(return-from small-prime-number-p nil)))
t)))
</CODE_SNIPPET> </CODE_SNIPPET>
<p> <p>
You should include a space between the semicolon and the text of the comment. You should include a space between the semicolon and the text of the comment.
@ -1564,7 +1619,7 @@ Robert Brown
</STYLEPOINT> </STYLEPOINT>
<STYLEPOINT title="Recursion"> <STYLEPOINT title="Recursion">
<SUMMARY> <SUMMARY>
Favor iteration over recursion. You should favor iteration over recursion.
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p> <p>
@ -1620,6 +1675,27 @@ Robert Brown
and only in cases where it won't surprise the person reading the code, and only in cases where it won't surprise the person reading the code,
and where it offers significant benefits. and where it offers significant benefits.
</p> </p>
<p>
Indeed, each special variable constitutes state.
Developers have to mentally track the state of all relevant variables
when trying to understand what the code does and how it does it;
tests have to be written and run with all relevant combinations;
to isolate some activity, care has to be taken to locally bind
all relevant variables, including those of indirectly used modules.
They can hide precious information from being printed in a backtrace.
Not only is there overhead associated to each new variable,
but interactions between variables
can make the code exponentially more complex
as the number of such variables increases.
The benefits have to match the costs.
</p>
<p>
Note though that a Lisp special variable is not a global variable
in the sense of a global variable in, say, BASIC or C.
As special variables can be dynamically bound to a local value,
they are much more powerful than
global value cells where all users necessarily interfere with each other.
</p>
<p> <p>
Good candidates for such special variables Good candidates for such special variables
are items for which "the current" can be naturally used as prefix, are items for which "the current" can be naturally used as prefix,
@ -1640,22 +1716,20 @@ Robert Brown
that these functions also have this argument, that these functions also have this argument,
which clutters the code with boilerplate. which clutters the code with boilerplate.
</p> </p>
<p>
Note that a Lisp special variable is not a global variable
in the sense of a global variable in, say, BASIC or C.
As special variables can be dynamically bound to a local value,
they are much more powerful than
global value cells where users all interfere with each other.
</p>
<p> <p>
You should treat special variables You should treat special variables
as though they are per-thread variables. as though they are per-thread variables.
That is, leave the special variable with no top-level binding at all, By default, you should leave a special variable
with no top-level binding at all,
and each thread of control and each thread of control
that needs the variable should bind it explicitly. that needs the variable should bind it explicitly.
This will mean that any incorrect use of the variable This will mean that any incorrect use of the variable
will result in an "unbound variable" error, and will result in an "unbound variable" error, and
each thread will see its own value for the variable. each thread will see its own value for the variable.
Variables with a default global value should usually be
locally bound at thread creation time.
You should use suitable infrastructure
to automate the appropriate declaration of such variables.
</p> </p>
</BODY> </BODY>
@ -1817,7 +1891,7 @@ Robert Brown
Code should not signal conditions from inside the cleanup form of Code should not signal conditions from inside the cleanup form of
<code>UNWIND-PROTECT</code> <code>UNWIND-PROTECT</code>
(unless they are always handled inside the cleanup form), (unless they are always handled inside the cleanup form),
or otherwise do non-local exits from cleanup handers or otherwise do non-local exits from cleanup handlers
outside of the handler e.g. <code>INVOKE-RESTART</code>. outside of the handler e.g. <code>INVOKE-RESTART</code>.
</li> </li>
<li> <li>
@ -2015,7 +2089,15 @@ Robert Brown
which might be a symbol or something. which might be a symbol or something.
</p> </p>
<p> <p>
You must not use MOP "intercessory" operations. You must not use MOP "intercessory" operations at runtime.
You should not use MOP "intercessory" operations at compile-time.
At runtime, they are at worst a danger, at best a performance issue.
At compile-time, it is usually cleaner that
macros should things up the right way on the first pass,
without the need to modify things in a second pass.
MOP intercession is a great tool for interactive development,
and you may enjoy it while debugging;
but you should not use it in normal applications.
</p> </p>
<p> <p>
If a class definition creates a method If a class definition creates a method
@ -2238,7 +2320,7 @@ Robert Brown
that generates code to do this. that generates code to do this.
See also <code>ALEXANDRIA:WITH-GENSYMS</code>, See also <code>ALEXANDRIA:WITH-GENSYMS</code>,
to make some temporary variables in the generated code. to make some temporary variables in the generated code.
Note that if you follow our <code>CALL-WITH-</code> style, Note that if you follow our <code>CALL-WITH</code> style,
you typically expand the code only once, as either you typically expand the code only once, as either
an argument to the auxiliary function, or an argument to the auxiliary function, or
the body of a lambda passed as argument to it; the body of a lambda passed as argument to it;
@ -2450,12 +2532,55 @@ Robert Brown
</p> </p>
</BODY> </BODY>
</STYLEPOINT> </STYLEPOINT>
<STYLEPOINT title="INTERN and UNINTERN">
<SUMMARY>
You must not use <code>INTERN</code> or <code>UNINTERN</code> at runtime.
</SUMMARY>
<BODY>
<p>
You must not use <code>INTERN</code> at runtime.
Not only does it cons,
it either creates a permanent symbol that won't be collected
or gives access to internal symbols.
This creates opportunities for memory leaks, denial of service attacks,
unauthorized access to internals, clashes with other symbols.
</p>
<p>
You must not <code>INTERN</code> a string
just to compare it to a keyword;
use <code>STRING=</code> or <code>STRING-EQUAL</code>.
</p>
<BAD_CODE_SNIPPET>
(member (intern str :keyword) $keys) ; Bad
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
(member str $keys :test #'string-equal) ; Better
</CODE_SNIPPET>
<p>
You must not use <code>UNINTERN</code> at runtime.
It can break code that relies on dynamic binding.
It makes things harder to debug.
You must not dynamically intern any new symbol,
and therefore you need not dynamically unintern anything.
</p>
<p>
You may of course use <code>INTERN</code> at compile-time,
in the implementation of some macros.
Even so, it is usually more appropriate
to use abstractions on top of it, such as
<code>ALEXANDRIA:SYMBOLICATE</code> or
<code>ALEXANDRIA:FORMAT-SYMBOL</code>
to create the symbols you need.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY> </CATEGORY>
<CATEGORY title="Data Representation"> <CATEGORY title="Data Representation">
<STYLEPOINT title="NIL: empty-list, false and I Don't Know"> <STYLEPOINT title="NIL: empty-list, false and I Don't Know">
<SUMMARY> <SUMMARY>
Appropriately use or avoid to use <code>NIL</code>. Appropriately use or avoid using <code>NIL</code>.
</SUMMARY> </SUMMARY>
<BODY> <BODY>
<p> <p>
@ -2699,67 +2824,6 @@ Robert Brown
</p> </p>
</BODY> </BODY>
</STYLEPOINT> </STYLEPOINT>
<STYLEPOINT title="#'FUN vs. 'FUN">
<SUMMARY>
You should usually refer to a function as <code>#'FOO</code> rather than <code>'FOO</code>.
</SUMMARY>
<BODY>
<p>
The former, which reads as <code>(FUNCTION FOO)</code>,
refers to the function object, and is properly scoped.
The latter, which reads as <code>(QUOTE FOO)</code>,
refers to the symbol, which when called
uses the global <code>FDEFINITION</code> of the symbol.
</p>
<p>
When using functions that take a functional argument
(e.g., <code>MAPCAR</code>, <code>APPLY</code>,
<code>:TEST</code> and <code>:KEY</code> arguments),
you should use the <code>#'</code> to quote the function,
not just single quote.
</p>
<p>
An exception is when you explicitly want dynamic linking,
because you anticipate that
the global function binding will be updated.
</p>
<p>
Another exception is when you explicitly want to access
a global function binding,
and avoid a possible shadowing lexical binding.
This shouldn't happen often, as it is usually a bad idea
to shadow a function when you will want to use the shadowed function;
just use a different name for the lexical function.
</p>
<p>
You must consistently use either <code>#'(lambda ...)</code>
or <code>(lambda ...)</code> without <code>#'</code> everywhere.
Unlike the case of <code>#'symbol</code> vs <code>'symbol</code>,
it is only a syntactic difference with no semantic impact,
except that the former works on Genera and the latter doesn't.
You must use the former style if your code is intended as a library
with maximal compatibility to all Common Lisp implementations;
otherwise, it is optional which style you use.
<code>#'</code> may be seen as a hint
that you're introducing a function in expression context;
but the <code>lambda</code> itself is usually sufficient hint,
and concision is good.
Choose wisely, but above all,
consistently with yourself and other developers,
within a same file, package, system, project, etc.
</p>
<p>
Note that if you start writing a new system
in a heavily functional style,
you may consider using
<a href="http://cliki.net/lambda-reader">lambda-reader</a>,
a system that lets you use the unicode character <code>&#955;</code>
instead of <code>LAMBDA</code>.
But you must not start using such a syntactic extension
in an existing system without getting permission from other developers.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY> </CATEGORY>
<CATEGORY title="Proper Forms"> <CATEGORY title="Proper Forms">
@ -2923,11 +2987,21 @@ Robert Brown
<p> <p>
Note that in Common Lisp, Note that in Common Lisp,
<code>WHEN</code> and <code>UNLESS</code> return <code>NIL</code> <code>WHEN</code> and <code>UNLESS</code> return <code>NIL</code>
when the condition evaluates to <code>NIL</code>. when the condition is not met.
You may take advantage of it.
Nevertheless, you may use an <code>IF</code> Nevertheless, you may use an <code>IF</code>
to explicitly return <code>NIL</code> to explicitly return <code>NIL</code>
if you have a specific reason to insist on the return value. if you have a specific reason to insist on the return value.
</p> </p>
<p>
You should include a fall-through clause <code>(t nil)</code>
as the last in your <cond>COND</cond>,
or <code>(otherwise nil)</code> as the last in your <cond>CASE</cond>
when the value returned by the conditional matters
and such a case is going to be used.
You should omit the fall-through clause
when the conditional is used for side-effects.
</p>
<p> <p>
You should prefer <code>AND</code> and <code>OR</code> You should prefer <code>AND</code> and <code>OR</code>
when it leads to more concise code than using when it leads to more concise code than using
@ -3232,49 +3306,6 @@ Robert Brown
</dl> </dl>
</BODY> </BODY>
</STYLEPOINT> </STYLEPOINT>
<STYLEPOINT title="INTERN and UNINTERN">
<SUMMARY>
You must not use <code>INTERN</code> or <code>UNINTERN</code> at runtime.
</SUMMARY>
<BODY>
<p>
You must not use <code>INTERN</code> it at runtime.
Not only does it cons,
it either creates a permanent symbol that won't be collected
or gives access to internal symbols.
This creates opportunities for memory leaks, denial of service attacks,
unauthorized access to internals, clashes with other symbols.
</p>
<p>
You must not <code>INTERN</code> a string
just to compare it to a keyword;
use <code>STRING=</code> or <code>STRING-EQUAL</code>.
</p>
<BAD_CODE_SNIPPET>
(member (intern str :keyword) $keys) ; Bad
</BAD_CODE_SNIPPET>
<CODE_SNIPPET>
(member str $keys :test #'string-equal) ; Better
</CODE_SNIPPET>
<p>
You must not use <code>UNINTERN</code> at runtime.
It can break code that relies on dynamic binding.
It makes things harder to debug.
You must not dynamically intern any new symbol,
and therefore you need not dynamically unintern anything.
</p>
<p>
You may of course use <code>INTERN</code> at compile-time,
in the implementation of some macros.
Even so, it is usually more appropriate
to use abstractions on top of it, such as
<code>ALEXANDRIA:SYMBOLICATE</code> or
<code>ALEXANDRIA:FORMAT-SYMBOL</code>
to create the symbols you need.
</p>
</BODY>
</STYLEPOINT>
</CATEGORY> </CATEGORY>
<CATEGORY title="Optimization"> <CATEGORY title="Optimization">
@ -3590,6 +3621,67 @@ Robert Brown
</CATEGORY> </CATEGORY>
<CATEGORY title="Pitfalls"> <CATEGORY title="Pitfalls">
<STYLEPOINT title="#'FUN vs. 'FUN">
<SUMMARY>
You should usually refer to a function as <code>#'FOO</code> rather than <code>'FOO</code>.
</SUMMARY>
<BODY>
<p>
The former, which reads as <code>(FUNCTION FOO)</code>,
refers to the function object, and is properly scoped.
The latter, which reads as <code>(QUOTE FOO)</code>,
refers to the symbol, which when called
uses the global <code>FDEFINITION</code> of the symbol.
</p>
<p>
When using functions that take a functional argument
(e.g., <code>MAPCAR</code>, <code>APPLY</code>,
<code>:TEST</code> and <code>:KEY</code> arguments),
you should use the <code>#'</code> to quote the function,
not just single quote.
</p>
<p>
An exception is when you explicitly want dynamic linking,
because you anticipate that
the global function binding will be updated.
</p>
<p>
Another exception is when you explicitly want to access
a global function binding,
and avoid a possible shadowing lexical binding.
This shouldn't happen often, as it is usually a bad idea
to shadow a function when you will want to use the shadowed function;
just use a different name for the lexical function.
</p>
<p>
You must consistently use either <code>#'(lambda ...)</code>
or <code>(lambda ...)</code> without <code>#'</code> everywhere.
Unlike the case of <code>#'symbol</code> vs <code>'symbol</code>,
it is only a syntactic difference with no semantic impact,
except that the former works on Genera and the latter doesn't.
You must use the former style if your code is intended as a library
with maximal compatibility to all Common Lisp implementations;
otherwise, it is optional which style you use.
<code>#'</code> may be seen as a hint
that you're introducing a function in expression context;
but the <code>lambda</code> itself is usually sufficient hint,
and concision is good.
Choose wisely, but above all,
consistently with yourself and other developers,
within a same file, package, system, project, etc.
</p>
<p>
Note that if you start writing a new system
in a heavily functional style,
you may consider using
<a href="http://cliki.net/lambda-reader">lambda-reader</a>,
a system that lets you use the unicode character <code>&#955;</code>
instead of <code>LAMBDA</code>.
But you must not start using such a syntactic extension
in an existing system without getting permission from other developers.
</p>
</BODY>
</STYLEPOINT>
<STYLEPOINT title="Pathnames"> <STYLEPOINT title="Pathnames">
<SUMMARY> <SUMMARY>
Common Lisp pathnames are tricky. Be aware of pitfalls. Common Lisp pathnames are tricky. Be aware of pitfalls.
@ -3780,7 +3872,7 @@ Robert Brown
</small> </small>
<p align="right"> <p align="right">
Revision 1.12 Revision 1.13
</p> </p>