Add makeAbsolutePath to MonadNix

Nix/Eval.hs

@@ -49,7 +49,7 @@ eval (NSym var) = do
 
 eval (NConstant x)    = return $ NVConstant x
 eval (NStr str)       = evalString str
-eval (NLiteralPath p) = return $ NVLiteralPath p
+eval (NLiteralPath p) = NVLiteralPath <$> makeAbsolutePath p
 eval (NEnvPath p)     = return $ NVEnvPath p
 
 eval (NUnary op arg) = arg >>= \case

Nix/Monad.hs

@@ -144,6 +144,9 @@ class (Show (NScopes m), MonadFix m) => MonadNix m where
     -- | Import a path into the nix store, and return the resulting path
     addPath :: FilePath -> m StorePath
 
+    -- | Determine the absolute path of relative path in the current context
+    makeAbsolutePath :: FilePath -> m FilePath
+
 deferInScope :: MonadNix m
              => NScopes m -> m (NValue m) -> m (NThunk m)
 deferInScope scope = buildThunk . clearScopes . pushScopes scope

Nix/Monad/Instance.hs

@@ -25,6 +25,7 @@ import           Nix.Parser
 import           Nix.Pretty
 import           Nix.Scope
 import           Nix.Utils
+import           System.Directory
 import           System.Environment
 import           System.Exit (ExitCode (ExitSuccess))
 import           System.FilePath
@@ -117,6 +118,13 @@ instance MonadNix (Cyclic IO) where
             return $ StorePath $ dropTrailingLinefeed out
           _ -> error $ "No such file or directory: " ++ show path
 
+    makeAbsolutePath p = if isAbsolute p then pure p else do
+        cwd <- lookupVar "__cwd" >>= \case
+            Nothing -> liftIO getCurrentDirectory
+            Just (NVLiteralPath s) -> return s
+            Just v -> throwError $ "when resolving relative path, __cwd is in scope, but is not a path; it is: " ++ show (void v)
+        liftIO $ canonicalizePath $ cwd </> p
+
     data NThunk (Cyclic IO) = NThunkIO (IORef (Deferred (Cyclic IO)))
 
     valueRef value =