%************************************************************************
%*                                                                      *
\section[recomp]{The GHC recompilation checker (and using `make')}
\index{recompilation checker}
\index{make and recompilation}
%*                                                                      *
%************************************************************************

The Haskell~1.3 module system (nicely improved, thank you very much)
requires a substantially different implementation, which we have done
for GHC~2.01.

We have taken a fairly radical approach and implemented a ``go to the
horse's mouth'' scheme; that is, when seeking out information about an
entity \tr{Foo.bar}, GHC will always look in the interface for
\tr{Foo}---no matter what interface {\em told us} about \tr{Foo.bar}
(via re-exporting and whatnot).

This ``horse's mouth'' scheme removes some of the most unsavoury
aspects of GHC~0.2x's behaviour (because of picking up duff
information about re-exported entities).  However, the keen-minded
user will see that it makes it difficult to maintain
\tr{make}-friendly dependencies between modules.

Enter the ``recompilation checker'', which you invoke merely by adding
a \tr{-recomp} option\index{-recomp option} to your GHC command line.
(You should also create ordinary `make' dependencies; there's an
example of how to do this in \sectionref{mkdependHS}.)

GHC will now start ``compiling'' much more often than in the old days,
but it will frequently bail out quickly, saying the recompile is
\tr{NOT NEEDED}.  What a beautiful sight!

The recompilation checker is not finished.  Its main faults are:
(a)~it doesn't yet do the right things for instance declarations;
(b)~it doesn't do anything about pragmas (happily, GHC~2.01 doesn't
produce any); (c)~it has no good solution for mutually-recursive
modules.

Patrick Sansom has a workshop paper about how all these things should
be done.  Ask him (email: \tr{sansom}) if you want a copy.