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+{- |
+Module      : Text.Pandoc.Class.Sandbox
+Copyright   : Copyright (C) 2021 John MacFarlane
+License     : GNU GPL, version 2 or above
+
+Maintainer  : John MacFarlane (<jgm@berkeley.edu>)
+Stability   : alpha
+Portability : portable
+
+This module provides a way to run PandocMonad actions in a sandbox
+(pure context, with no IO allowed and access only to designated files).
+-}
+
+module Text.Pandoc.Class.Sandbox
+  ( sandbox )
+where
+
+import Control.Monad (foldM)
+import Control.Monad.Except (throwError)
+import Control.Monad.IO.Class (MonadIO (liftIO))
+import Text.Pandoc.Class.PandocMonad
+import Text.Pandoc.Class.PandocPure
+import Text.Pandoc.Class.CommonState (CommonState(..))
+import Text.Pandoc.Logging (messageVerbosity)
+
+-- | Lift a PandocPure action into any instance of PandocMonad.
+-- The main computation is done purely, but CommonState is preserved
+-- continuously, and warnings are emitted after the action completes.
+-- The parameter is a list of FilePaths which will be added to the
+-- ersatz file system and be available for reading.
+sandbox :: (PandocMonad m, MonadIO m) => [FilePath] -> PandocPure a -> m a
+sandbox files action = do
+  oldState <- getCommonState
+  tree <- liftIO $ foldM addToFileTree mempty files
+  case runPure (do putCommonState oldState
+                   modifyPureState $ \ps -> ps{ stFiles = tree }
+                   result <- action
+                   st <- getCommonState
+                   return (st, result)) of
+          Left e -> throwError e
+          Right (st, result) -> do
+            putCommonState st
+            let verbosity = stVerbosity st
+            -- emit warnings, since these are not printed in runPure
+            let newMessages = reverse $ take
+                  (length (stLog st) - length (stLog oldState)) (stLog st)
+            mapM_ logOutput
+              (filter ((<= verbosity) . messageVerbosity) newMessages)
+            return result
+