[GHC] #9813: Error when reifying type constructor

[GHC]

#9813: Error when reifying type constructor
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        Reporter:  owst              |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  7.8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1899
       Wiki Page:                    |
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Comment (by goldfire):

 Replying to [comment:22 owst]:
 > Are `f` and `g` mutually recursive? Since you explicitly say `g`
 ''cannot'' reify `f` - does `f` therefore just depend on `g`?

 We declare that they are. That's what's meant by "anything with a splice
 depends on anything else". It's true that the splice can't do much reify
 `f`, but it can certainly produce code which calls `f`.

 >
 > What about in:
 > {{{
 > f = e $(...)
 > c = $(...)
 > e = c ...
 > d = $(...)
 > a = ... b ...
 > b = ... a ...
 > }}}
 >
 > I think an processing order: `(a b) < (c d) < e < f` could work:


 I don't think this would quite work. What if `c` calls `f`? That is, `c =
 $(varE (mkName (['a'..] !! 5 : [])))`. We don't know without running the
 splice, so we say that anything with a splice depends on anything else in
 the region.

 >
 >
 > > It does strike me (inspired by comment:16) that with all this
 toposorting going on, we may be able to avoid having top-level splices
 break the file up into declaration groups. The current behavior seems
 something like a feeble attempt at a toposort, anyway.
 >
 > Indeed, I think that would be nice for a consistent story. However, how
 do you order top-level declarations in the toposort?

 Perhaps I was confusing. Sorry. I changed my mind as I wrote my response
 -- in the end, I don't think this would work, because it would assume that
 expression splices above the top-level splice could depend on declarations
 below the top-level splice, which might break existing code (and with no
 workaround).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9813#comment:23>
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