id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
1604	Coarse-grained recompilation checking	simonmar		"GHC's recompilation checker is very clever: it tracks changes at the level of an individual function or type, and only recompiles a module when the entities it actually depends on have changed.

However, there are some drawbacks to doing this:

 * It is complicated and hard to get right
 * It adds extra information to .hi files
 * It takes time to check the dependencies (probably not much, though)

Furthermore,

 * we have virtually no tests for it
 * when it goes wrong, the bug is very hard to reproduce, which means that
   failures are often not reported.

So we propose:

 * Simplify the recompilation checker so that it tracks changes at the level of a
   module only.  This will cause more recompilation, but it will be much easier
   to get right and to test.
 * As an intermediate step, we could implement both schemes and generate some output
   to indicate when they give different results.

See [wiki:Commentary/Compiler/RecompilationAvoidance] for more discussion."	task	closed	normal	6.10 branch	Compiler	6.6.1	wontfix			Unknown/Multiple	Unknown/Multiple		Unknown