Looking up importDecl and Unique inequality

[Moritz Angermann]

Hi *,

I'm still trying to get a stage1 compiler with TH support.  After some experimenting, I managed to 
adjust ghc-7.8 sufficiently to build and not complain about compiling TH code right away.  I'm now
stuck with importDecl trying to import Language.Haskell.TH.Lib.ExpQ (as that is referenced in my
sample code I try to compile), but even after loading the TH/Lib.hi file unable to find it due to a
missmatch in the Uniques.

I have a Stage 2, ghc-7.8 (A) compiler for the host, which I use to compile a Stage 1 ghc-7.8 (B) compiler.
Now I want to inject some code (a plugin) into (B), which therefore is compiled with (A) and hence depends
on the packages of (A).  But as A and B are the identical version, I hope that I should be able to feed (B)
the same package db.

Given that the plugin was compiled into a cabal package with (A) at (Y) and the package db of (A) is at (X),
I try to compile my sample code with

$ cabal exec B -- path/to/Sample.hs -package-db X -package-db Y -package plugin -fplugin MyPlugin

  -- Sample.hs --
  {-# LANGUAGE TemplateHaskell #-}
  module Main where
  
  main :: IO ()
  main = do
    let e = $([|Just "Splice!"|]) :: Maybe String
    putStrLn . show $ e
  -- Sample.hs --

The type checker tries to find Language.Haskell.TH.Lib.ExpQ and loads the lib/TH.hi from (X) with
the declaration, but fails to find it in the eps_PTE after loading the interface.

What I was able to figure out was that Language.Haskell.TH.Lib.ExpQ loaded from lib/TH.hi in (X) obtains the
unique key "rCM", while the unique key that is being looked up is "39S".

Can someone shed some light onto where I am misunderstanding how something is supposed to work?

Cheers,
 Moritz