Run tests as doctests

Setup.hs

@@ -1,3 +1,8 @@
+{-# LANGUAGE CPP #-}
+{-# OPTIONS_GHC -Wall #-}
+
+module Main (main) where
+
 import Distribution.PackageDescription (
   HookedBuildInfo,
   emptyHookedBuildInfo )
@@ -10,11 +15,40 @@ import Distribution.Simple.Setup ( SDistFlags )
 
 import System.Process ( system )
 
+#ifndef MIN_VERSION_cabal_doctest
+#define MIN_VERSION_cabal_doctest(x,y,z) 0
+#endif
 
+#if MIN_VERSION_cabal_doctest(1,0,0)
+
+import Distribution.Extra.Doctest ( addDoctestsUserHook )
+main :: IO ()
+main = defaultMainWithHooks $ addDoctestsUserHook "doctests" myHooks
+  where
+    myHooks = simpleUserHooks { preSDist = myPreSDist }
+
+#else
+
+#ifdef MIN_VERSION_Cabal
+-- If the macro is defined, we have new cabal-install,
+-- but for some reason we don't have cabal-doctest in package-db
+--
+-- Probably we are running cabal sdist, when otherwise using new-build
+-- workflow
+#warning You are configuring this package without cabal-doctest installed. \
+         The doctests test-suite will not work as a result. \
+         To fix this, install cabal-doctest before configuring.
+#endif
+
+main :: IO ()
 main = defaultMainWithHooks myHooks
   where
     myHooks = simpleUserHooks { preSDist = myPreSDist }
 
+#endif
+
+
+
 -- | This hook will be executed before e.g. @cabal sdist@. It runs
 --   pandoc to create the man page from shellcheck.1.md. If the pandoc
 --   command is not found, this will fail with an error message: