Merge pull request #286 from jwiegley/merge257

Pull request for merge257

src/Nix/Builtins.hs

@@ -134,6 +134,7 @@ builtinsList = sequence [
     , add2 Normal   "div"                        div_
     , add2 Normal   "elem"                       elem_
     , add2 Normal   "elemAt"                     elemAt_
+    , add  Normal   "exec"                       exec_
     , add0 Normal   "false"                      (return $ nvConstant $ NBool False)
     , add  Normal   "fetchTarball"               fetchTarball
     , add2 Normal   "filter"                     filter_
@@ -190,6 +191,7 @@ builtinsList = sequence [
     , add  Normal   "toPath"                     toPath
     , add  TopLevel "toString"                   toString
     , add  Normal   "toXML"                      toXML_
+    , add2 TopLevel "trace"                      trace_
     , add  Normal   "tryEval"                    tryEval
     , add  Normal   "typeOf"                     typeOf
     , add  Normal   "unsafeDiscardStringContext" unsafeDiscardStringContext
@@ -855,6 +857,17 @@ tryEval e = catch (onSuccess <$> e) (pure . onError)
         , ("value", valueThunk (nvConstant (NBool False)))
         ]
 
+trace_ :: forall e m. MonadNix e m => m (NValue m) -> m (NValue m) -> m (NValue m)
+trace_ msg action = do
+  traceEffect . Text.unpack =<< fromValue @Text msg
+  action
+
+exec_ :: forall e m. MonadNix e m => m (NValue m) -> m (NValue m)
+exec_ xs = do
+  ls <- fromValue @[NThunk m] xs
+  xs <- traverse (fromValue @Text . force') ls
+  exec (map Text.unpack xs)
+
 fetchTarball :: forall e m. MonadNix e m => m (NValue m) -> m (NValue m)
 fetchTarball v = v >>= \case
     NVSet s _ -> case M.lookup "url" s of

src/Nix/Effects.hs

@@ -32,3 +32,7 @@ class MonadFile m => MonadEffects m where
     nixInstantiateExpr :: String -> m (NValue m)
 
     getRecursiveSize :: a -> m (NValue m)
+
+    traceEffect :: String -> m ()
+
+    exec :: [String] -> m (NValue m)

src/Nix/Exec.hs

@@ -583,6 +583,27 @@ instance (MonadFix m, MonadCatch m, MonadIO m, Alternative m,
         const $ toNix (0 :: Integer)
 #endif
 
+    traceEffect = liftIO . putStrLn
+
+    exec = \case
+      [] -> throwError $ ErrorCall "exec: missing program"
+      (prog:args) -> do
+        (exitCode, out, _) <-
+            liftIO $ readProcessWithExitCode prog args ""
+        let t = Text.strip (Text.pack out)
+        let emsg = "program[" ++ prog ++ "] args=" ++ show args
+        case exitCode of
+          ExitSuccess ->
+            if Text.null t
+            then throwError $ ErrorCall $ "exec has no output :" ++ emsg
+            else case parseNixTextLoc t of
+              Failure err ->
+                throwError $ ErrorCall $
+                    "Error parsing output of exec: " ++ show err ++ " " ++ emsg
+              Success v -> evalExprLoc v
+          err -> throwError $ ErrorCall $
+                    "exec  failed: " ++ show err ++ " " ++ emsg
+
 runLazyM :: Options -> MonadIO m => Lazy m a -> m a
 runLazyM opts = (`evalStateT` M.empty)
               . (`runReaderT` newContext opts)