[Add auto-generated CPP macros for package version testing
Simon Marlow <marlowsd@gmail.com>**20080811173016
 
 Now when using CPP you get 
 
    MIN_VERSION_<package>(A,B,C)
 
 for each <package> in build-depends, which is true if the version of
 <package> in use is >= A.B.C, using the normal ordering on version
 numbers.
 
 This is done by auto-generating a header file
 dist/build/autogen/cabal_macros.h, and passing a -include flag when
 running CPP.
] {
hunk ./Distribution/Simple/Build.hs 65
-         ( Package(..), packageName, packageVersion )
+         ( PackageIdentifier, Package(..), packageName, packageVersion )
hunk ./Distribution/Simple/Build.hs 74
-import Distribution.Simple.BuildPaths ( autogenModuleName, autogenModulesDir )
+import Distribution.Simple.BuildPaths
+                                ( autogenModulesDir, autogenModuleName,
+                                  cppHeaderName )
hunk ./Distribution/Simple/Build.hs 82
+import Distribution.Version     ( versionBranch )
hunk ./Distribution/Simple/Build.hs 89
+import Text.Printf              ( printf )
hunk ./Distribution/Simple/Build.hs 160
+  buildCPPHeader distPref pkg_descr lbi
hunk ./Distribution/Simple/Build.hs 164
+-- ------------------------------------------------------------
+-- * Building cabal_macros.h
+-- ------------------------------------------------------------
+
+buildCPPHeader :: FilePath -> PackageDescription -> LocalBuildInfo -> IO ()
+buildCPPHeader distPref _pkg_descr lbi =
+   let  
+	cpp_header_filepath = autogenModulesDir lbi </> cppHeaderName
+
+        preface = "/* DO NOT EDIT: This file is automatically generated by Cabal */"
+
+        version_macro :: PackageIdentifier -> String
+        version_macro pkgid = 
+            printf ("#define MIN_VERSION_%s(major1,major2,minor) \\\n" ++
+                    "  (major1) < %d || \\\n" ++
+                    "  (major1) == %d && (major2) < %d || \\\n" ++
+                    "  (major1) == %d && (major2) == %d && (minor) <= %d")
+                   (display (packageName pkgid)) major1 
+                                                 major1 major2
+                                                 major1 major2 minor
+            where
+              vs = versionBranch (packageVersion pkgid)
+              (major1:major2:minor:_) = vs ++ repeat 0
+
+        contents = unlines (preface : map version_macro (packageDeps lbi))
+
+   in do 
+      btime <- getModificationTime (localBuildInfoFile distPref)
+      exists <- doesFileExist cpp_header_filepath
+      ptime <- if exists
+   	            then getModificationTime cpp_header_filepath
+   	            else return btime
+      if btime >= ptime
+	   then writeFile cpp_header_filepath contents
+	   else return ()
+
hunk ./Distribution/Simple/BuildPaths.hs 49
+    cppHeaderName,
hunk ./Distribution/Simple/BuildPaths.hs 95
+cppHeaderName :: String
+cppHeaderName = "cabal_macros.h"
hunk ./Distribution/Simple/GHC.hs 676
+     ++ ["-I" ++ autogenModulesDir lbi]
hunk ./Distribution/Simple/GHC.hs 681
+     ++ [ "-optP-include", "-optP"++ (autogenModulesDir lbi </> cppHeaderName) ]
hunk ./Distribution/Simple/PreProcess.hs 74
-import Distribution.Simple.BuildPaths (autogenModulesDir)
+import Distribution.Simple.BuildPaths (autogenModulesDir,cppHeaderName)
hunk ./Distribution/Simple/PreProcess.hs 322
+       ++ [ "-optP-include", "-optP"++ (autogenModulesDir lbi </> cppHeaderName) ]
hunk ./Distribution/Simple/PreProcess.hs 334
-      rawSystemProgramConf verbosity cpphsProgram (withPrograms lbi) $
+      rawSystemProgram verbosity cpphsProg $
hunk ./Distribution/Simple/PreProcess.hs 337
-        : extraArgs
+        : (if cpphsVersion >= Version [1,6] []
+             then ["--include="++ (autogenModulesDir lbi </> cppHeaderName)]
+             else [])
+        ++ extraArgs
hunk ./Distribution/Simple/PreProcess.hs 342
+  where Just cpphsProg = lookupProgram cpphsProgram (withPrograms lbi)
+        Just cpphsVersion = programVersion cpphsProg
hunk ./doc/Cabal.xml 1684
+    <sect2 id="cpp">
+      <title>Conditional compilation</title>
+
+      <para>Sometimes you want to write code that works with more than
+      one version of a dependency.  You can specify a range of
+        versions for the depenency in
+      the <literal>build-depends</literal>, but how do you then write
+        the code that can use different versions of the API?</para>
+      
+      <para>Haskell lets you preprocess your code using the C
+        preprocessor (either the real C preprocessor, or
+        <literal>cpphs</literal>).  To enable this,
+        add <literal>extensions: CPP</literal> to your package
+        description.  When using CPP, Cabal provides some pre-defined
+        macros to let you test the version of dependent packages; for
+        example, suppose your package works with either version 3 or
+        version 4 of the <literal>base</literal> package, you could
+        select the available version in your Haskell modules like
+        this:</para> 
+<programlisting>
+#if MIN_VERSION_base(4,0,0)
+... code that works with base-4 ...
+#else
+... code that works with base-3 ...
+#endif
+</programlisting>
+<para>In general, Cabal supplies a
+  macro <literal>MIN_VERSION_<replaceable>package</replaceable>(A,B,C)</literal>
+  for each package depended on via <literal>build-depends</literal>.
+  This macro is true if the actual version of the package in use is
+  greater than or equal to <literal>A.B.C</literal> (using the
+  conventional ordering on version numbers, which is lexicographic on
+  the sequence, but numeric on each component, so for example 1.2.0 is
+  greater than 1.0.3).</para>
+
+      <para>Cabal places the definitions of these macros into an
+        automatically-generated header file, which is included when
+        preprocessing Haskell source code by passing options to the C
+        preprocessor.</para>
+    </sect2>
+
}