base-feature-macros: Semantic CPP feature macros for base

This provides a set of feature macros describing features of base in a semantic way.

See <base-feature-macros.h> for the set of currently provided macros.

In order to use the CPP header provided by this package, add this package as a dependency to your .cabal file, i.e.

build-depends: base-feature-macros: >= 0.1 && < 0.2

while making sure that the version specified as lower bound defines the feature-macros your code tests for. This is particularly important as CPP will implicitly treat undefined CPP macros as having the value 0. See also GNU CPP/CC's -Wundef warning to detect such errors; or starting with GHC 8.2, -Wcpp-undef can be used:

if impl(ghc >= 8.2)
    ghc-options: -Wcpp-undef

Then in your code, you can include and use the <base-feature-macros.h> header like so

module M where

#include <base-feature-macros.h>

#if !HAVE_FOLDABLE_TRAVERSABLE_IN_PRELUDE
import Data.Foldable (Foldable (..))
import Prelude       hiding (foldr, foldr1)
#endif
#if !HAVE_MONOID_IN_PRELUDE
import Data.Monoid hiding ((<>))
#endif

This package is inspired by the blogpost &quot;Make macros mean something &#8211; readable backwards compatibility with CPP&quot;.