[GHC] #2555: Template Haskell does not respect -package and -hide constraints

[GHC]

#2555: Template Haskell does not respect -package and -hide constraints
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    Reporter:  guest    |       Owner:          
        Type:  bug      |      Status:  new     
    Priority:  normal   |   Component:  Compiler
     Version:  6.8.2    |    Severity:  normal  
    Keywords:           |    Testcase:          
Architecture:  Unknown  |          Os:  Linux   
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 (I am using 6.8.2-2ubuntu1, packaged for Ubuntu Hardy Heron, on amd64.)

 Recently sjanssen added some patches to the Lambdabot darcs repository
 which replace a preprocessor, BotPP, with some Template Haskell. This
 seems to work well, however I ran into a problem with it - everytime TH
 was called, it would error out with a message about Bytestring 0.9.1.0.

 This is far from the first time I've run into this problem, but it was not
 fixed by forcibly removing 0.9.1.0 & recompiling affected packages against
 bytestring 09.0.1!

 After even more debugging, I finally figured it out:

 The module TH was erroring out on has an 'import Text.Regex'. Text.Regex
 is installed by the regex-compat package. I had two regex-compats
 installed. regex-compat-0.91 was linked against bytestring0.9.0.1. regex-
 compat-0.92 was linked against (the removed) bytestring-0.9.1.0.

 Now, normally Cabal detects that using 0.92 would lead to errors, and it
 selects 0.91, and it adds to the GHC options '-hide-all-package -package
 regex-compat-0.91', and everything compiles fine and the user is happy.

 But! in this case, Template Haskell ignores the flags, and it uses the
 most recent regex-compat. And it is this that breaks the build. I
 ultimately resolved this issue by removing regex-compat-0.92 and
 reinstalling and linking it against 0.9.0.1, although I fear this may yet
 lead to bugs in the future.

 Short summary: Cabal is smarter than TH and picks better package versions
 to compile against, and yet TH ignores Cabal's picks. This leads to
 unnecessary & difficult to figure out compile failures.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2555>
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