Add a new --values option for selectively displaying value provenance

main/Main.hs

@@ -89,6 +89,8 @@ main = do
                              <=< fromNix
                     | normalize opts =
                       liftIO . print . prettyNixValue <=< normalForm
+                    | values opts  =
+                      liftIO . print <=< renderNValueProv
                     | otherwise  =
                       liftIO . print <=< renderNValue
 

src/Nix/Options.hs

@@ -12,6 +12,7 @@ data Options = Options
     { verbose      :: Verbosity
     , tracing      :: Bool
     , thunks       :: Bool
+    , values       :: Bool
     , reduce       :: Maybe FilePath
     , reduceSets   :: Bool
     , reduceLists  :: Bool
@@ -43,6 +44,7 @@ defaultOptions = Options
     { verbose      = ErrorsOnly
     , tracing      = False
     , thunks       = False
+    , values       = False
     , reduce       = Nothing
     , reduceSets   = False
     , reduceLists  = False
@@ -109,6 +111,9 @@ nixOptions = Options
     <*> switch
         (   long "thunks"
          <> help "Enable reporting of thunk tracing as well as regular evaluation")
+    <*> switch
+        (   long "values"
+         <> help "Enable reporting of value provenance in error messages")
     <*> optional (strOption
         (   long "reduce"
          <> help "When done evaluating, output the evaluated part of the expression to FILE"))

src/Nix/Pretty.hs

@@ -260,7 +260,10 @@ renderNValueF :: MonadVar m => NValueF m (NThunk m) -> m Doc
 renderNValueF = fmap prettyNixValue . removeEffectsM
 
 renderNValue :: MonadVar m => NValue m -> m Doc
-renderNValue = \case
+renderNValue (NValue _ v) = renderNValueF v
+
+renderNValueProv :: MonadVar m => NValue m -> m Doc
+renderNValueProv = \case
     NValue [] v -> renderNValueF v
     NValue ps v -> do
         v' <- renderNValueF v

src/Nix/Render/Frame.hs

@@ -137,10 +137,16 @@ renderValueFrame level = pure . (:[]) . \case
 
 renderExecFrame :: (MonadReader e m, Has e Options, MonadVar m, MonadFile m)
                 => NixLevel -> ExecFrame m -> m [Doc]
-renderExecFrame _level = fmap (:[]) . \case
-    Assertion v ->
-        -- jww (2018-04-24): Render values nicely based on the verbosity.
-        (text "Assertion failed:" </>) <$> renderNValue v
+renderExecFrame _level f = do
+    opts :: Options <- asks (view hasLens)
+    (:[]) <$> case f of
+        Assertion v
+            | values opts ->
+                  -- jww (2018-04-24): Render value provenance differently
+                  -- based on the verbosity.
+                  (text "Assertion failed:" </>) <$> renderNValueProv v
+            | otherwise ->
+                  pure $ text "Assertion failed"
 
 renderThunkLoop :: (MonadReader e m, Has e Options, MonadFile m)
                 => NixLevel -> ThunkLoop -> m [Doc]