Hide stack support behind --stack flag

main/Main.hs

@@ -103,6 +103,8 @@ parseKernelArgs = foldl' addFlag defaultKernelSpecOptions
       kernelSpecOpts { kernelSpecGhcLibdir = libdir }
     addFlag kernelSpecOpts (KernelspecInstallPrefix prefix) =
       kernelSpecOpts { kernelSpecInstallPrefix = Just prefix }
+    addFlag kernelSpecOpts KernelspecUseStack =
+      kernelSpecOpts { kernelSpecUseStack = True }
     addFlag kernelSpecOpts flag = error $ "Unknown flag" ++ show flag
 
 -- | Run the IHaskell language kernel.
@@ -112,6 +114,7 @@ runKernel :: KernelSpecOptions -- ^ Various options from when the kernel was ins
 runKernel kernelOpts profileSrc = do
   let debug = kernelSpecDebug kernelOpts
       libdir = kernelSpecGhcLibdir kernelOpts
+      useStack = kernelSpecUseStack kernelOpts
 
   -- Parse the profile file.
   Just profile <- liftM decode $ LBS.readFile profileSrc
@@ -121,6 +124,7 @@ runKernel kernelOpts profileSrc = do
   Stdin.recordKernelProfile dir profile
 
 #if MIN_VERSION_ghc(7,8,0)
+  when useStack $ do
     -- Detect if we have stack
     runResult <- try $ readProcessWithExitCode "stack" [] ""
     let stack = 

src/IHaskell/Flags.hs

@@ -38,6 +38,7 @@ data Argument = ConfFile String     -- ^ A file with commands to load at startup
               | ConvertToFormat NotebookFormat
               | ConvertLhsStyle (LhsStyle String)
               | KernelspecInstallPrefix String
+              | KernelspecUseStack
   deriving (Eq, Show)
 
 data LhsStyle string =
@@ -100,6 +101,11 @@ kernelDebugFlag = flagNone ["debug"] addDebug "Print debugging output from the k
   where
     addDebug (Args mode prev) = Args mode (KernelDebug : prev)
 
+kernelStackFlag :: Flag Args
+kernelStackFlag = flagNone ["stack"] addStack "Inherit environment from `stack` when it is installed"
+  where
+    addStack (Args mode prev) = Args mode (KernelspecUseStack : prev)
+
 confFlag :: Flag Args
 confFlag = flagReq ["conf", "c"] (store ConfFile) "<rc.hs>"
              "File with commands to execute at start; replaces ~/.ihaskell/rc.hs."
@@ -118,11 +124,11 @@ store constructor str (Args mode prev) = Right $ Args mode $ constructor str : p
 installKernelSpec :: Mode Args
 installKernelSpec =
   mode "install" (Args InstallKernelSpec []) "Install the Jupyter kernelspec." noArgs
-    [ghcLibFlag, kernelDebugFlag, confFlag, installPrefixFlag, helpFlag]
+    [ghcLibFlag, kernelDebugFlag, confFlag, installPrefixFlag, helpFlag, kernelStackFlag]
 
 kernel :: Mode Args
 kernel = mode "kernel" (Args (Kernel Nothing) []) "Invoke the IHaskell kernel." kernelArg
-           [ghcLibFlag, kernelDebugFlag, confFlag]
+           [ghcLibFlag, kernelDebugFlag, confFlag, kernelStackFlag]
   where
     kernelArg = flagArg update "<json-kernel-file>"
     update filename (Args _ flags) = Right $ Args (Kernel $ Just filename) flags

src/IHaskell/IPython.hs

@@ -47,6 +47,7 @@ data KernelSpecOptions =
          , kernelSpecDebug :: Bool                 -- ^ Spew debugging output?
          , kernelSpecConfFile :: IO (Maybe String) -- ^ Filename of profile JSON file.
          , kernelSpecInstallPrefix :: Maybe String
+         , kernelSpecUseStack :: Bool              -- ^ Whether to use @stack@ environments.
          }
 
 defaultKernelSpecOptions :: KernelSpecOptions
@@ -55,6 +56,7 @@ defaultKernelSpecOptions = KernelSpecOptions
   , kernelSpecDebug = False
   , kernelSpecConfFile = defaultConfFile
   , kernelSpecInstallPrefix = Nothing
+  , kernelSpecUseStack = False
   }
 
 -- | The IPython kernel name.
@@ -188,6 +190,7 @@ installKernelspec replace opts = void $ do
            Nothing   -> []
            Just file -> ["--conf", file])
         ++ ["--ghclib", kernelSpecGhcLibdir opts]
+        ++ ["--stack" | kernelSpecUseStack opts]
 
   let kernelSpec = KernelSpec
         { kernelDisplayName = "Haskell"