GHC's CPP and Cabal's unlit

[Alistair Bayley]

I'm doing some testing with GHC 6.6.1 and Cabal 1.3, and I'm trying to
figure out what happens with CPP and Cabal's unlit.

I start with file Test.lhs:

> {-# OPTIONS -fglasgow-exts #-}
> module Test where
> main = putStrLn "hello CPP"

and run command:
  ghc -E -x hs -cpp Test.lhs -o Test2.lhs

which gives me Test2.lhs:

{-# LINE 1 "Test.lhs" #-}
# 1 "Test.lhs"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "Test.lhs"
> {-# OPTIONS -fglasgow-exts #-}
> module Test where
> main = putStrLn "hello CPP"


So I'm wondering: where does the {-# LINE #-} comment come from, and
also the # 1 lines? AFAICT the # 1 lines are ignored by GHC; I can
compile Test2.lhs without errors. Is there anything in GHC's docs
about this?

More puzzling is that the files that Cabal runs through ghc's CPP
don't get the # n lines, so we end up with something like this:

{-# LINE 1 "Test.lhs" #-}
> {-# OPTIONS -fglasgow-exts #-}
> module Test where
> main = putStrLn "hello CPP"

which is not a valid .lhs file, because we have a code line next to a comment.

I also note in Cabal the haddock command runs CPP before unlit. GHC
does it the other way around i.e. run unlit first then CPP, and I'm
wondering if Cabal shouldn't do the same thing?

Thanks,
Alistair