add doctest.

ghc-mod.cabal

@@ -87,6 +87,15 @@ Executable ghc-mod
                       , ghc
                       , ghc-mod
 
+Test-Suite doctest
+  Type:                 exitcode-stdio-1.0
+  Default-Language:     Haskell2010
+  HS-Source-Dirs:       test
+  Ghc-Options:          -threaded -Wall
+  Main-Is:              doctests.hs
+  Build-Depends:        base
+                      , doctest >= 0.9.3
+
 Test-Suite spec
   Default-Language:     Haskell2010
   Main-Is:              Spec.hs

test/doctests.hs

@@ -0,0 +1,10 @@
+module Main where
+
+import Test.DocTest
+
+main :: IO ()
+main = doctest [
+    "-package"
+  , "ghc"
+  , "Language/Haskell/GhcMod.hs"
+  ]