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Coarse-grained recompilation checking

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 Commentary/Compiler/RecompilationAvoidance for more discussion.

Edited by Simon Marlow
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