add doctest runner

IHaskell.cabal

@@ -89,3 +89,10 @@ executable IHaskell
                        here,
                        system-filepath,
                        text ==0.11.*
+
+Test-Suite doctests
+  Type:     exitcode-stdio-1.0
+  Ghc-Options: -threaded
+  Main-Is: rundoctests.hs
+  Build-Depends: base, doctest >= 0.8, process
+

rundoctests.hs

+import System.Process
+import System.Exit
+import System.IO
+import Test.DocTest
+import Data.Char
+import System.Environment
+
+-- | tests that all the >>> comments are followed by correct output. Easiest is to
+--
+-- > cabal test
+--
+-- or
+--
+-- > runghc examples/rundoctests.hs
+--
+-- or
+--
+-- > runghc examples/rundoctests.hs Data/HList/File1.hs Data/HList/File2.hs
+--
+-- you need Cabal >= 1.18 since that's around when cabal repl got added.
+main = do
+    as <- getArgs
+    o <- readProcess
+        "cabal" ["repl","--ghc-options","-v0 -w"]
+        ":show packages\n:show language"
+    let flags = words $ unlines $ filter ((=="-") . take 1 . dropWhile isSpace)
+                    $ lines o
+
+    let files = case as of
+            [] -> ["Main.hs"]
+            _ -> as
+
+    doctest $ "-i.": "-idist/build/autogen":
+             "-optP-include":
+             "-optPdist/build/autogen/cabal_macros.h" :
+             "-Idist/build/autogen" : "-w":
+             files ++ flags