refactor stack vm to use a basic block ish structure

test/doctest.hs

@@ -1,7 +1,17 @@
+{-# LANGUAGE DoAndIfThenElse #-}
 module Main where
-
+
 import Test.DocTest (mainFromCabal)
-import System.Environment (getArgs)
+import System.Environment (getArgs, lookupEnv)
+import System.IO (stderr, hPutStrLn)
+
 
 main :: IO ()
-main = mainFromCabal "gyehoek" =<< getArgs
+main = do
+  nix <- maybe False null <$> lookupEnv "GYEHOEK_IN_NIX_BUILD"
+  if nix then do
+    hPutStrLn stderr "\
+     \skipping doctests in Nix build environment. \
+     \see https://github.com/pcapriotti/optparse-applicative/pull/408."
+  else
+    mainFromCabal "gyehoek" =<< getArgs