[GHC] #8610: Rebuild on a definition-based granularity

[GHC]

#8610: Rebuild on a definition-based granularity
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       Reporter:  EyalLotem         |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:  _|_
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Currently, GHC rebuilds modules whose mtime suggests they changed, and all
 dependent modules.

 This rebuilds *far* more than is actually necessary due to the change.

 For example, if a single definition changed in a module, only that
 definition needs to be rebuilt, and very few of the definitions in the
 dependent modules probably depend on it, so only they ought to be rebuilt.

 Another example, is temporarily checking out a different branch, and then
 going back to the master branch. In this case, the mtimes of many modules
 will change, but the content will remain the same.

 If definition-granularity changes were detected, rather than module-
 granularity, this scenario will rebuild nothing.

 This improvement could shorten the development cycle of all Haskell
 projects significantly, making Haskell a significantly more productive
 language to use on larger projects.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8610>
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