Setup.hs

import Control.Monad
import Distribution.Simple
import System.Environment
--import System.IO


-- Surprisingly, if the user types "cabal install --enable-tests",
-- `args` will *not* be ["install", "--enable-tests"]. Instead, cabal will
-- run Setup.hs repeatedly with different arguments:
-- 
--     ["configure","--verbose=1","--builddir=dist/dist-sandbox-28d8356a","--ghc","--prefix=...",...]
--     ["build","--verbose=1","--builddir=dist/dist-sandbox-28d8356a"]
--     ["test","--builddir=dist/dist-sandbox-28d8356a"]
--     ["install","--verbose=1","--builddir=dist/dist-sandbox-28d8356a"]
-- 
-- We need to manipulate `args` via `substitute` in order to preserve those
-- extra arguments.
substitute :: Eq a => [(a, [a])] -> [a] -> [a]
substitute substitutions = (>>= go)
  where
    go x = case lookup x substitutions of
             Just xs -> xs
             Nothing -> return x

main = do
    args <- getArgs
    
    --withFile "Setup.log" AppendMode $ \h -> do
    --  hPutStrLn h (show args)
    
    when ("test" `elem` args) $ do
      -- unlike most packages, this one needs to be installed before it can be tested.
      defaultMainArgs (substitute [ ("test", ["install","--verbose=1"])
                                  
                                  -- remove test-specific arguments
                                  , ("--log=$pkgid-$test-suite.log", [])
                                  , ("--machine-log=$pkgid.log", [])
                                  , ("--show-details=failures", [])
                                  ] args)
    
    defaultMainArgs args