Re: [GHC] #12723: Family instance modules are not fingerprinted in ABI

17 Oct 2016


      #12723: Family instance modules are not fingerprinted in ABI
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Driver            |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by ezyang:

@@ -44,1 +44,1 @@
- decide not to get recompiled? Because the ABI hash of B did not change.
+ decide not to get recompiled? Because the ABI hash of C did not change.

New description:

 This leads to the following delightful, five module bug:

 {{{
 -- A.hs
 {-# LANGUAGE TypeFamilies #-}
 module A where
 type family F a
 type instance F Int = Bool

 -- B.hs
 module B where
 import A

 -- C.hs
 module C where
 import B

 -- D.hs
 module D where
 import C

 -- E.hs
 module E where
 import D
 import B
 }}}

 Build these modules. Then rename A to A2 (fixing B's import), and build
 them again. You'll get:

 {{{
 ezyang@sabre:~/Dev/labs/T3871$ ghc --make E.hs
 [1 of 5] Compiling A2               ( A2.hs, A2.o )
 [2 of 5] Compiling B                ( B.hs, B.o )
 [3 of 5] Compiling C                ( C.hs, C.o ) [B changed]
 [5 of 5] Compiling E                ( E.hs, E.o ) [B changed]
 attempting to use module ‘A’ (./A.hs) which is not loaded
 }}}

 The problem is clear: D was not recompiled, but it needs to be, because
 when the module gets renamed, we need to update its list of family
 instance modules to rename A to A2. When we don't do this, the subsequent
 family instance check chokes because it tries to load A. And why did D
 decide not to get recompiled? Because the ABI hash of C did not change.
 And that's WRONG.

 (Also, family instances are really awful, you really do have to rebuild
 everything when you change them. UGH.)

--

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12723#comment:1
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