On Mon, 5 Oct 2015, Gregory Collins wrote:
Um, no, it usually isn't anything like that. Here's a sampling of some of the things I've used CPP for in the past few years: * After GHC 7.4, when using a newtype in FFI imports you need to import the constructor, i.e. "import Foreign.C.Types(CInt(..))" --- afaik CPP is the only way to shut up warnings everywhere
I import them qualified and then define type CInt = CTypes.CInt sometimes.
* defaultTimeLocale moved from System.Locale to Data.Time.Format in time-1.5 (no compat package for this, afaik)
I was advised to always import time-1.5.
* one of many various changes to Typeable in the GHC 7.* series (deriving works better now, mkTyCon vs mkTyCon3, etc)
I have not used that. Like I have not used the ever changing Template Haskell stuff.
* Do I have to hide "catch" from Prelude, or not? It got moved, and "hiding" gives an error if the symbol you're trying to hide is missing. Time to break out the CPP (and curse myself for not just using the qualified import in the first place)
I think System.IO.Error.catchIOError maintains the old behaviour.
* Do I get monoid functions from Prelude or from Data.Monoid? Same w/ Applicative, Foldable, Word. I don't know where anything is supposed to live anymore, or which sequence of imports will shut up spurious warnings on all four versions of GHC I support, so the lowest-friction fix is: break out the #ifdef spackle
I always import them from the most specific module. Where GHC-7.10 Prelude causes conflicts I even started to import more basic Prelude stuff from modules like Data.Bool, Data.Eq and friends. Horrible, but works without CPP.
* ==# and friends return Int# instead of Bool after GHC 7.8.1
I did not use it. I have also warned that the change might not be a good idea since LLVM knows that its 'i1' type can only have the values 0 and 1 and it does not know that for 'i32' or 'i64'.
* To use functions like "tryReadMVar", "unsafeShiftR", and "atomicModifyIORef'" that are in recent base versions but not older ones (this is a place where CPP use is actually justified)
I have not used them so far. I have solved more complicated cases with conditional Hs-Source-Dirs: http://wiki.haskell.org/Cabal/Developer-FAQ#Adapt_to_different_systems_witho... It's cumbersome but so far I managed most transitions without CPP. (Nonetheless, MRP would require the complicated Hs-Source-Dirs solution for far too many packages.)