id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
877	Template Haskell doesn't parse data decls properly	brianh@…		"Hi,
Template Haskell is rejecting data decls just because of the particular identifier I'm choosing to use. The code is:
{{{
   module Box where

   data T = T Int
}}}
{{{
{-# OPTIONS_GHC -fglasgow-exts -fth #-}
module Main where

import qualified Box as Box

$( [d| data T = T Box.T |] )

main = return ()
}}}
And the error message is:
{{{
main.hs:6:18: Not in scope: type constructor or class `Box.T'
}}}
If I change the decl in the splice to:
{{{
    $( [d| data Hello = T Box.T |] )
}}}
the code compiles with no problems! (???)
I thought I could workaround the issue by adding
{{{
    type T = Hello
}}}
but although this workaround works in the simple example above, in my actual program I get a {{{ghc panic}}} error that I don't know how to reproduce in a simple example.

The problem seems to be that I'm using the name {{{T}}} for both types, but I'd prefer to do this as it's consistent with the rest of my code (this coding style is recommended by Henning at the bottom of the page  [http://haskell.org/hawiki/UsingQualifiedNames])

The only workaround seems to be to declare a type synonym for Box.T ie:
{{{
  type BoxT = Box.T

  $( [d| data T = T BoxT |] )
}}}
but while this is ok it's not ideal as then type synonyms have to be defined in each module instead of just being able to use the neat qualified naming syntax. Also it is strange because the normal decl
{{{
     data T = T Box.T
}}}
works fine, so decls in [d| |] brackets must be being parsed differently from normal top level decls for some reason (I'm new to TH so I don't know why).

(The reason I'm doing this is to get extra fields added automatically to a record (and methods to an instance decl) to implement one level of inlined OO implementation inheritance in Haskell)

Anyway it's not essential, just a bit strange. I'll use the BoxT synonym for the moment.

Regards, Brian."	bug	closed	normal		Compiler	6.4.2	fixed	Template TH parsing identifier		Windows	x86		Unknown