add builtins.toFile

src/Nix/Builtins.hs

@@ -194,6 +194,7 @@ builtinsList = sequence [
     , add' Normal   "toJSON"
       (arity1 $ decodeUtf8 . LBS.toStrict . A.encodingToLazyByteString
                            . toEncodingSorted)
+    , add2 Normal   "toFile"                     toFile
     , add  Normal   "toPath"                     toPath
     , add  TopLevel "toString"                   toString
     , add  Normal   "toXML"                      toXML_
@@ -660,6 +661,13 @@ functionArgs fun = fun >>= \case
     v -> throwError $ ErrorCall $
             "builtins.functionArgs: expected function, got " ++ show v
 
+toFile :: MonadNix e m => m (NValue m) -> m (NValue m) -> m (NValue m)
+toFile name s = do
+    name' <- fromValue name
+    s' <- fromValue s
+    mres <- toFile_ (Text.unpack name') (Text.unpack s')
+    toNix $ Text.pack $ unStorePath mres
+
 toPath :: MonadNix e m => m (NValue m) -> m (NValue m)
 toPath = fromValue @Path >=> toNix @Path
 

src/Nix/Effects.hs

@@ -13,6 +13,8 @@ class MonadFile m => MonadEffects m where
     -- | Import a path into the nix store, and return the resulting path
     addPath :: FilePath -> m StorePath
 
+    toFile_ :: FilePath -> String -> m StorePath
+
     -- | Determine the absolute path of relative path in the current context
     makeAbsolutePath :: FilePath -> m FilePath
     findEnvPath :: String -> m FilePath
@@ -38,3 +40,4 @@ class MonadFile m => MonadEffects m where
     traceEffect :: String -> m ()
 
     exec :: [String] -> m (NValue m)
+

src/Nix/Exec.hs

@@ -483,6 +483,12 @@ instance (MonadFix m, MonadCatch m, MonadIO m, Alternative m,
           _ -> throwError $ ErrorCall $
                   "addPath: failed: nix-store --add " ++ show path
 
+    toFile_ filepath content = do
+      liftIO $ writeFile filepath content
+      storepath <- addPath filepath
+      liftIO $ removeFile filepath
+      return storepath
+
     makeAbsolutePath origPath = do
         origPathExpanded <- liftIO $ expandHomePath origPath
         absPath <- if isAbsolute origPathExpanded then pure origPathExpanded else do