- <para>GHC keeps a version number on each interface file, and on
- each type signature within the interface file. It also keeps in
- every interface file a list of the version numbers of everything
- it used when it last compiled the file. If the source file's
- modification date is earlier than the <filename>.o</filename>
- file's date (i.e. the source hasn't changed since the file was
- last compiled), and the recompilation checking is on, GHC will be
- clever. It compares the version numbers on the things it needs
- this time with the version numbers on the things it needed last
- time (gleaned from the interface file of the module being
- compiled); if they are all the same it stops compiling rather
- early in the process saying “Compilation IS NOT
- required”. What a beautiful sight!</para>
-
- <para>Patrick Sansom had a workshop paper about how all this is
- done (though the details have changed quite a bit). <ulink
- url="mailto:sansom@dcs.gla.ac.uk">Ask him</ulink> if you want a
- copy.</para>
+ <para>GHC calculates a fingerprint (in fact an MD5 hash) of each
+ interface file, and of each declaration within the interface
+ file. It also keeps in every interface file a list of the
+ fingerprints of everything it used when it last compiled the
+ file. If the source file's modification date is earlier than
+ the <filename>.o</filename> file's date (i.e. the source hasn't
+ changed since the file was last compiled), and the recompilation
+ checking is on, GHC will be clever. It compares the fingerprints
+ on the things it needs this time with the fingerprints
+ on the things it needed last time (gleaned from the
+ interface file of the module being compiled); if they are all
+ the same it stops compiling early in the process saying
+ “Compilation IS NOT required”. What a beautiful
+ sight!</para>
+
+ <para>You can read
+ about <ulink url="http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/RecompilationAvoidance">how
+ all this works</ulink> in the GHC commentary.</para>