Re: [GHC] #1012: ghc panic with mutually recursive modules and template haskell

13 Oct 2013

      #1012: ghc panic with mutually recursive modules and template haskell
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        Reporter:  guest             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  lowest            |        Milestone:  7.6.2
       Component:  Template Haskell  |          Version:  6.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  TH_import_loop    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 I just got bitten by this, and I don't see a clean reorganization. While I
 agree that this should be "lowest" priority, I thought I'd document my
 real-life use case:

 The singletons library generates a fair amount of code using TH. In
 particular, it produces instances for classes, type families, and data
 families. Accordingly, the TH code needs to know the names of these
 classes and families, and the names of the class members. In the main
 module of the library, I want to use the TH code to produce lots of
 instances for datatypes exported from base (`Bool`, `Maybe`, `()`, ...). I
 don't want these instances to be orphans. So, I'm stuck.

 I'll illustrate further with some code:

 Module `Data.Singletons` (boot):
 {{{
 data family Sing (a :: k)
 }}}

 Module `Data.Singletons.Singletons`:
 {{{
 import {-# SOURCE #-} Data.Singletons

 singName :: Name
 singName = ''Sing

 genSingletons :: [Name] -> Q [Dec]
 genSingletons = ...
 }}}

 Module `Data.Singletons`:
 {{{
 import Data.Singletons.Singletons

 data family Sing (a :: k)
 $(genSingletons [''Bool, ''Maybe, ''Either, ''[]])
 }}}

 This all seems quite sensible to me, yet GHC can't do it. It seems I have
 several bad options: 1) allow orphans, defining the `Sing` family in a
 different module than its instances; 2) hand-write the instances I want,
 avoiding the loop; or 3) use `mkName` to create the names. I'm going to do
 (3), but it feels dirty and fragile.

 Note: I do see why calling a function at compile time that is defined in a
 module in a mutually-recursive group with the calling module could be very
 bad. But, I'm not quite doing that here. That said, I don't see an easy
 way to allow what I want, prohibit the bad behavior, and not pollute the
 syntax and implementation of it all. The reason for this comment is just
 to document a case where I'm a little hamstrung by this missing behavior,
 not that I really want resources dedicated to finding a fix here.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1012#comment:27
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