[GHC] #7373: When building GHC: Failed to load interface for `GHC.Fingerprint'

[GHC]

#7373: When building GHC: Failed to load interface for `GHC.Fingerprint'
-------------------------------+--------------------------------------------
  Reporter:  igloo             |          Owner:                  
      Type:  bug               |         Status:  new             
  Priority:  normal            |      Milestone:  7.8.1           
 Component:  libraries/base    |        Version:  7.6.1           
Resolution:                    |       Keywords:                  
        Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown      |     Difficulty:  Unknown         
  Testcase:  driver/T7373      |      Blockedby:                  
  Blocking:                    |        Related:                  
-------------------------------+--------------------------------------------

Comment(by simonpj):

 What are the "false differences", precisely?

 I understand what is happening now. In your example we have
  * Package `pkg` has
    * Module A depends on nothing
    * Module C, depends on A and {-# SOURCE #-} B
    * Module B, depends on C

  * Home package has
    * Module D depends on pkg:C

 Now, if you compile D, it needs to load C.hi, which in turn needs the
 interface for B.  But because it's not in the home package (the one to
 which the module D being compiled belongs), GHC assumes `pkg` is complete
 compiled and doesn't track which of `B.hi` or `B.hi-boot` should be
 available; it just looks for `B.hi`.  In the test, you delete `B.hi` hence
 the error.

 I'm rather inclined to say "too bad" here.  From outside, it's better to
 wait for the loop to be compiled, and ''then'' compile client packages.
 In the example, make D import B.  Or make D import B instead of C, and
 make B export all of C's exports.  Or make a third module CB that imports
 both C and B, and use that in client modules.

 This isn't just convenience.  You get better cross-module inlining once B
 is compiled, so you only want to use `B.hi-boot` when there is absolutely
 no alternative, ie while compiling modules that form part of the cycle.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7373#comment:11>
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