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The following code works under ghc-7.6.3 and template-haskell-2.8.0.0:

{-# LANGUAGE TemplateHaskell #-}

data Huh = ThisDefinitely
         | UsedToWork

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