[GHC] #9813: Error when reifying type constructor

19 Nov 2014

      #9813: Error when reifying type constructor
-------------------------------------+-------------------------------------
       Reporter:  owst               |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 The following code works under ghc-7.6.3 and template-haskell-2.8.0.0:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}

 data Huh = ThisDefinitely
          | UsedToWork

 constructorNames :: String
 constructorNames = $(do
     ty <- reify ''Huh
     let showCon (NormalC n _) = nameBase n
         strs = case ty of
                     (TyConI (DataD _ _ _ cons _)) -> map showCon cons
     return . LitE . StringL $ concat strs)

 main = putStrLn constructorNames
 }}}

 Printing the following at compile time:

 {{{
 [1 of 1] Compiling Main             ( Main.hs, Main.o )
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Loading package array-0.4.0.1 ... linking ... done.
 Loading package deepseq-1.3.0.1 ... linking ... done.
 Loading package containers-0.5.0.0 ... linking ... done.
 Loading package pretty-1.1.1.0 ... linking ... done.
 Loading package template-haskell ... linking ... done.
 Linking Main ...
 }}}

 and then successfully executing:

 {{{
 $ ./Main
 ThisDefinitelyUsedToWork
 }}}

 However, using ghc 7.8.3/template-haskell-2.9.0.0 I get the following
 compile error:
 {{{
 [1 of 1] Compiling Main             ( Main.hs, Main.o )
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 Loading package array-0.5.0.0 ... linking ... done.
 Loading package deepseq-1.3.0.2 ... linking ... done.
 Loading package containers-0.5.5.1 ... linking ... done.
 Loading package pretty-1.1.1.1 ... linking ... done.
 Loading package template-haskell ... linking ... done.

 Main.hs:8:22:
     ‘Huh’ is not in the type environment at a reify
     In the splice:
       $(do { ty <- reify ''Huh;
              let showCon (NormalC n _) = ...
                  ....;
              return . LitE . StringL $ concat strs })
 }}}

 Is this expected? I couldn't see anything in the GHC release notes to
 suggest this should no longer work.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9813
GHC http://www.haskell.org/ghc/
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