[Haskell-cafe] Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`

[Tom Ellis]

On Mon, Oct 05, 2015 at 05:40:43PM -0700, Gregory Collins wrote:
> On Mon, Oct 5, 2015 at 3:18 PM, Bryan Richter <b at chreekat.net> wrote:
> > Hang on a moment, are you saying that all the people writing to argue
> > that these changes would require them to write dozens more #ifdef's
> > actually don't have to write any at all?
> 
> 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
>    - defaultTimeLocale moved from System.Locale to Data.Time.Format in
>    time-1.5 (no compat package for this, afaik)
>    - one of many various changes to Typeable in the GHC 7.* series
>    (deriving works better now, mkTyCon vs mkTyCon3, etc)
>    - 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)
>    - 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
>    - ==# and friends return Int# instead of Bool after GHC 7.8.1
>    - 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)

In fact I think all of these apart from the FFI one could be solved with a
-compat package, could they not?