# Changes between Version 33 and Version 34 of Commentary/CodingStyle

Ignore:
Timestamp:
Apr 24, 2011 10:03:52 PM (6 years ago)
Comment:

more section restructuring

### Legend:

Unmodified
 v33 This is a rough description of some of the coding practices and style that we use for Haskell code inside {{{compiler}}}.  For run-time system code see the [wiki:Commentary/Rts/Conventions Coding Style Guidelines for RTS C code].  Also see the wiki page on [wiki:WorkingConventions Working Conventions] for issues related to version control, workflow, testing, bug tracking and other miscellany. == General Style == The general rule is to stick to the same coding style as is already used in the file you're editing. If you must make stylistic changes, commit them separately from functional changes, so that someone looking back through the change logs can easily distinguish them. It's much better to write code that is transparent than to write code that is short. Often it's better to write out the code longhand than to reuse a generic abstraction (not always, of course).  Sometimes it's better to duplicate some similar code than to try to construct an elaborate generalisation with only two instances.  Remember: other people have to be able to quickly understand what you've done, and overuse of abstractions just serves to obscure the ''really'' tricky stuff, and there's no shortage of that in GHC. == Comments == pragma; you are encouraged to remove this pragma and fix any warnings when working on a module. == Literate Haskell == In GHC we use a mixture of literate ({{{.lhs}}}) and non-literate ({{{.hs}}}) source. I (Simon M.) prefer to use non-literate style, because I think the {{{\begin{code}..\end{code}}}} clutter up the source too much, and I like to use Haddock-style comments (we haven't tried processing the whole of GHC with Haddock yet, though). == The C Preprocessor (CPP) == Currently we pass all the compiler sources through CPP. The -cpp flag is always added by the build system. However, whenever possible we try to avoid using CPP, as it can hide code from the compiler (which means changes that work on one platform can break the build on another) and code using CPP can be harder to understand. The following CPP symbols are used throughout the compiler: '''DEBUG''':: Used to enables extra checks and debugging output in the compiler. The ASSERT macro (see {{{HsVersions.h}}}) provides assertions which disappear when DEBUG is not defined. However, whenever possible, it is better to us debugIsOn from the Util module, which is defined to be True when DEBUG is defined and False otherwise.  The ideal way to provide debugging output is to use a Haskell expression "when debugIsOn $..." to arrange that the compiler will be silent when DEBUG is off (unless of course something goes wrong or the verbosity level is nonzero). When option -O is used, GHC will easily sweep away the unreachable code. As a last resort, debugging code can be placed inside #ifdef DEBUG, but since this strategy guarantees that only a fraction of the code is seen be the compiler on any one compilation, it is to be avoided when possible. Regarding performance, a good rule of thumb is that DEBUG shouldn't add more than about 10-20% to the compilation time. This is the case at the moment. If it gets too expensive, we won't use it. For more expensive runtime checks, consider adding a flag - see for example -dcore-lint. '''Trap, pitfall for using the ASSERT macro''': The ASSERT macro uses CPP, and if you are unwise enough to try to write assertions using primed variables ({{{ASSERT (not$ intersectsBlockEnv b b')}}}), one possible outcome is that CPP silently fails to expand the ASSERT, and you get this very baffling error message: {{{ Not in scope: data constructor ASSERT' }}} Now you can Google for this error message :-) '''GHCI''':: Enables GHCi support, including the byte code generator and interactive user interface. This isn't the default, because the compiler needs to be bootstrapped with itself in order for GHCi to work properly. The reason is that the byte-code compiler and linker are quite closely tied to the runtime system, so it is essential that GHCi is linked with the most up-to-date RTS. Another reason is that the representation of certain datatypes must be consistent between GHCi and its libraries, and if these were inconsistent then disaster could follow. == Compiler versions and language extensions == GHC must be compilable by the previous two major releases, and itself. It isn't necessary for it to be compilable by every intermediate development version (that includes last week's darcs sources). To maintain compatibility, use [wiki:Commentary/CodingStyle#HsVersions.h HsVersions.h] (see below) where possible, and try to avoid using #ifdef in the source itself. Also, it is necessary to avoid certain language extensions.  In particular, the {{{ScopedTypeVariables}}} extension must not be used. == Walk-through of a sample source file == We now describe a typical source file, annotating stylistic choices as we go. ===  The OPTIONS pragma === An {{{{-# OPTIONS_GHC ... #-}}}} pragma is optional, but if present it should go right at the top of the file. Things you might want to put in OPTIONS include: * {{{#include}}} options to bring into scope prototypes for FFI declarations * {{{-fvia-C}}} if you know that this module won't compile with the native code generator.  (deprecated: everything should compile with the NCG nowadays, but that wasn't always the case). Don't bother putting -cpp or -fglasgow-exts in the OPTIONS pragma; these are already added to the command line by the build system. == Exports and Imports == === Exports === It's helpful to give type signatures inside comments in the export list, but hard to keep them consistent, so we don't always do that. === {{{HsVersions.h}}} === {{{HsVersions.h}}} is a CPP header file containing a number of macros that help smooth out the differences between compiler versions. It defines, for example, macros for library module names which have moved between versions. Take a look [[GhcFile(compiler/HsVersions.h)]]. {{{ #include "HsVersions.h" }}} === Imports === If the module can be compiled multiple ways (eg. GHCI vs. non-GHCI), make sure the imports are properly #ifdefed too, so as to avoid spurious unused import warnings. === General Style === It's much better to write code that is transparent than to write code that is short. Often it's better to write out the code longhand than to reuse a generic abstraction (not always, of course).  Sometimes it's better to duplicate some similar code than to try to construct an elaborate generalisation with only two instances.  Remember: other people have to be able to quickly understand what you've done, and overuse of abstractions just serves to obscure the ''really'' tricky stuff, and there's no shortage of that in GHC. == Literate Haskell == In GHC we use a mixture of literate ({{{.lhs}}}) and non-literate ({{{.hs}}}) source. I (Simon M.) prefer to use non-literate style, because I think the {{{\begin{code}..\end{code}}}} clutter up the source too much, and I like to use Haddock-style comments (we haven't tried processing the whole of GHC with Haddock yet, though). == The C Preprocessor (CPP) == Currently we pass all the compiler sources through CPP. The -cpp flag is always added by the build system. However, whenever possible we try to avoid using CPP, as it can hide code from the compiler (which means changes that work on one platform can break the build on another) and code using CPP can be harder to understand. The following CPP symbols are used throughout the compiler: '''DEBUG''':: Used to enables extra checks and debugging output in the compiler. The ASSERT macro (see {{{HsVersions.h}}}) provides assertions which disappear when DEBUG is not defined. However, whenever possible, it is better to us debugIsOn from the Util module, which is defined to be True when DEBUG is defined and False otherwise.  The ideal way to provide debugging output is to use a Haskell expression "when debugIsOn $..." to arrange that the compiler will be silent when DEBUG is off (unless of course something goes wrong or the verbosity level is nonzero). When option -O is used, GHC will easily sweep away the unreachable code. As a last resort, debugging code can be placed inside #ifdef DEBUG, but since this strategy guarantees that only a fraction of the code is seen be the compiler on any one compilation, it is to be avoided when possible. Regarding performance, a good rule of thumb is that DEBUG shouldn't add more than about 10-20% to the compilation time. This is the case at the moment. If it gets too expensive, we won't use it. For more expensive runtime checks, consider adding a flag - see for example -dcore-lint. '''Trap, pitfall for using the ASSERT macro''': The ASSERT macro uses CPP, and if you are unwise enough to try to write assertions using primed variables ({{{ASSERT (not$ intersectsBlockEnv b b')}}}), one possible outcome is that CPP silently fails to expand the ASSERT, and you get this very baffling error message: {{{ Not in scope: data constructor ASSERT' }}} Now you can Google for this error message :-) '''GHCI''':: Enables GHCi support, including the byte code generator and interactive user interface. This isn't the default, because the compiler needs to be bootstrapped with itself in order for GHCi to work properly. The reason is that the byte-code compiler and linker are quite closely tied to the runtime system, so it is essential that GHCi is linked with the most up-to-date RTS. Another reason is that the representation of certain datatypes must be consistent between GHCi and its libraries, and if these were inconsistent then disaster could follow. == Compiler versions and language extensions == GHC must be compilable by the previous two major releases, and itself. It isn't necessary for it to be compilable by every intermediate development version (that includes last week's darcs sources). To maintain compatibility, use [wiki:Commentary/CodingStyle#HsVersions.h HsVersions.h] (see below) where possible, and try to avoid using #ifdef in the source itself. Also, it is necessary to avoid certain language extensions.  In particular, the {{{ScopedTypeVariables}}} extension must not be used. ===  The OPTIONS pragma === An {{{{-# OPTIONS_GHC ... #-}}}} pragma is optional, but if present it should go right at the top of the file. Things you might want to put in OPTIONS include: * {{{#include}}} options to bring into scope prototypes for FFI declarations * {{{-fvia-C}}} if you know that this module won't compile with the native code generator.  (deprecated: everything should compile with the NCG nowadays, but that wasn't always the case). Don't bother putting -cpp or -fglasgow-exts in the OPTIONS pragma; these are already added to the command line by the build system. === {{{HsVersions.h}}} === {{{HsVersions.h}}} is a CPP header file containing a number of macros that help smooth out the differences between compiler versions. It defines, for example, macros for library module names which have moved between versions. Take a look [[GhcFile(compiler/HsVersions.h)]]. {{{ #include "HsVersions.h" }}}