[GHC] #9813: Error when reifying type constructor

[GHC]

#9813: Error when reifying type constructor
-------------------------------------+-------------------------------------
              Reporter:  owst        |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Description changed by owst:

Old description:

> 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.

New description:

 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 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.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9813#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler