Re: [GHC] #13803: Panic while forcing the thunk for TyThing IsFile (regression)

11 Jun 2017

      #13803: Panic while forcing the thunk for TyThing IsFile (regression)
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        Reporter:  inaki             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:  hs-boot
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 So, here is a test case which is not solved by the test above:

 {{{
 -- F.hs
 {-# LANGUAGE TypeFamilies #-}
 module F where
 type family F a :: *

 -- A.hs-boot
 module A where
 data T

 -- B.hs
 {-# LANGUAGE TypeFamilies #-}
 module B where
 import {-# SOURCE #-} A
 import F
 type instance F T = Int

 -- C.hs
 {-# LANGUAGE TypeFamilies #-}
 module C where
 import {-# SOURCE #-} A
 import F
 type instance F T = Bool

 -- A.hs
 module A where
 import B
 import C
 }}}

 Right now, we decide to defer a type family consistency check if the
 family was recursively defined.  If the RHS refers to a recursively
 defined type, there's no problem: we don't need to look at it for
 consistency checking. But if the LHS is recursively defined, as is in this
 example, we DO need to defer the check.

 But it's a bit irritating to figure out whether or not there's actually a
 reference to a recursively defined type in the LHS, since this involves
 traversing the LHS types, and if we're not careful we'll end up pulling in
 the TyThing anyway. There are two other possibilities: (1) always defer
 checking instances which are defined inside the recursive look (by looking
 at the Name of the axiom), or (2) annotating IfaceAxiom with the set of
 boot types its LHS refers to, for easy checking. Not entirely sure what
 the best action is.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13803#comment:15
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