1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
4 <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1">
5 <title>The GHC Commentary - Coding Style Guidelines</title>
8 <body BGCOLOR="FFFFFF">
9 <h1>The GHC Commentary - Coding Style Guidelines</h1>
11 <p>This is a rough description of some of the coding practices and
12 style that we use for Haskell code inside <tt>ghc/compiler</tt>.
14 <p>The general rule is to stick to the same coding style as is
15 already used in the file you're editing. If you must make
16 stylistic changes, commit them separately from functional changes,
17 so that someone looking back through the change logs can easily
20 <h2>To literate or not to literate?</h2>
22 <p>In GHC we use a mixture of literate (<tt>.lhs</tt>) and
23 non-literate (<tt>.hs</tt>) source. I (Simon M.) prefer to use
24 non-literate style, because I think the
25 <tt>\begin{code}..\end{code}</tt> clutter up the source too much,
26 and I like to use Haddock-style comments (we haven't tried
27 processing the whole of GHC with Haddock yet, though).
29 <h2>To CPP or not to CPP?</h2>
31 <p>We pass all the compiler sources through CPP. The
32 <tt>-cpp</tt> flag is always added by the build system.
34 <p>The following CPP symbols are used throughout the compiler:
37 <dt><tt>DEBUG</tt></dt>
39 <dd>Used to enables extra checks and debugging output in the
40 compiler. The <tt>ASSERT</tt> macro (see <tt>HsVersions.h</tt>)
41 provides assertions which disappear when <tt>DEBUG</tt> is not
44 <p>All debugging output should be placed inside <tt>#ifdef
45 DEBUG</tt>; we generally use this to provide warnings about
46 strange cases and things that might warrant investigation. When
47 <tt>DEBUG</tt> is off, the compiler should normally be silent
48 unless something goes wrong (exception when the verbosity level
49 is greater than zero).
51 <p>A good rule of thumb is that <tt>DEBUG</tt> shouldn't add
52 more than about 10-20% to the compilation time. This is the case
53 at the moment. If it gets too expensive, we won't use it. For
54 more expensive runtime checks, consider adding a flag - see for
55 example <tt>-dcore-lint</tt>.
58 <dt><tt>GHCI</tt></dt>
60 <dd>Enables GHCi support, including the byte code generator and
61 interactive user interface. This isn't the default, because the
62 compiler needs to be bootstrapped with itself in order for GHCi
63 to work properly. The reason is that the byte-code compiler and
64 linker are quite closely tied to the runtime system, so it is
65 essential that GHCi is linked with the most up-to-date RTS.
66 Another reason is that the representation of certain datatypes
67 must be consistent between GHCi and its libraries, and if these
68 were inconsistent then disaster could follow.
73 <p>More about what you're allowed to do in the way of CPP
76 <h2>Compiler versions</h2>
78 <p>GHC must be compilable by every major version of GHC from 4.08
79 onwards, and itself. It isn't necessary for it to be compilable
80 by every intermediate development version (that includes last
81 week's CVS sources), but we mustn't lose compatibility with 4.08
82 for the time being, because that's the only version which can be
83 easily bootstrapped from .hc files.
85 <p>To maintain compatibility, use <tt>HsVersions.h</tt> (see
86 below) where possible, and try to avoid using <tt>#ifdef</tt> in
89 <h2>The source file</h2>
91 <p>We now describe a typical source file, annotating stylistic
98 <p>An <tt>OPTIONS</tt> pragma is optional, but if present it
99 should go right at the top of the file. Things you might want to
100 put in <tt>OPTIONS</tt> include:
103 <li><tt>-#include</tt> options to bring into scope prototypes
104 for FFI declarations</li>
105 <li><tt>-fvia-C</tt> if you know that
106 this module won't compile with the native code generator.
109 <p>Don't bother putting <tt>-cpp</tt> or <tt>-fglasgow-exts</tt>
110 in the <tt>OPTIONS</tt> pragma; these are already added to the
111 command line by the build system.
121 <p>We usually (99% of the time) include an export list. The only
122 exceptions are perhaps where the export list would list absolutely
123 everything in the module, and even then sometimes we do it anyway.
125 <p>It's helpful to give type signatures inside comments in the
126 export list, but hard to keep them consistent, so we don't always
130 #include "HsVersions.h"
133 <p><tt>HsVersions.h</tt> is a CPP header file containing a number
134 of macros that help smooth out the differences between compiler
135 versions. It defines, for example, macros for library module
136 names which have moved between versions. Take a look.
144 import Id ( idName, idType )
148 import DATA_IOREF ( newIORef, readIORef )
151 import List ( partition )
152 import Maybe ( fromJust )
155 <p>List imports in the following order:
158 <li>Local to this subsystem (or directory) first</li>
160 <li>Compiler imports, generally ordered from specific to generic
161 (ie. modules from <tt>utils/</tt> and <tt>basicTypes/</tt>
162 usually come last)</li>
164 <li>Library imports</li>
166 <li>Standard Haskell 98 imports last</li>
169 <p>Import library modules from the <tt>base</tt> and
170 <tt>haskell98</tt> packages only. Use <tt>#defines</tt> in
171 <tt>HsVersions.h</tt> when the modules names differ between
172 versions of GHC (eg. <tt>DATA_IOREF</tt> in the example above).
173 For code inside <tt>#ifdef GHCI</tt>, don't need to worry about GHC
174 versioning (because we are bootstrapped).
176 <p>We usually use import specs to give an explicit list of the
177 entities imported from a module. The main reason for doing this is
178 so that you can search the file for an entity and find which module
179 it comes from. However, huge import lists can be a pain to
180 maintain, so we often omit the import specs when they start to get
181 long (actually I start omitting them when they don't fit on one
182 line --Simon M.). Tip: use GHC's <tt>-fwarn-unused-imports</tt>
183 flag so that you get notified when an import isn't being used any
186 <p>If the module can be compiled multiple ways (eg. GHCI
187 vs. non-GHCI), make sure the imports are properly <tt>#ifdefed</tt>
188 too, so as to avoid spurious unused import warnings.
190 <p><em>ToDo: finish this</em>