[GHC] #12035: hs-boot knot tying insufficient for ghc --make

[GHC]

#12035: hs-boot knot tying insufficient for ghc --make
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        Reporter:  ezyang            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:  hs-boot
      Resolution:                    |  backpack
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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Comment (by ezyang):

 This PDF gives the full story, http://web.mit.edu/~ezyang/Public/backpack-
 symbol-tables.pdf but the short answer is that there are two ways we loop
 tie hs-boot. The first, which is correctly implemented for `--make`, is
 retypechecking all the modules that depended on the `hs-boot` file. The
 second is tying the loop ''at the same time'' we are typechecking the `hs`
 module which implements the `hs-boot` file; i.e., it's what
 `tcg_type_env_var` is for. If we have:

 {{{
 -- A.hs-boot
 module A where
 x :: Bool

 -- B.hs
 module B where
 import {-# SOURCE #-} A
 y = not x

 -- A.hs
 module A where
 import B
 x = True
 z = not y
 }}}

 When we typecheck `A.hs`, does the unfolding for `y` have an up-to-date
 unfolding for `x`? In one-shot mode the answer is yes (and thus we see the
 behavior of #10083), but in make mode the answer is no. The retypecheck
 loop has nothing to do with it, since it happens AFTER we finish building
 the hs file.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12035#comment:2>
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