check-ppr: Add a --dump flag to aid in debugging

Currently tracking down where two ASTs disagree is quite difficult. Add a --dump
flag to check-ppr which dumps the respective ASTs to files, which can then be
easily compared with diff, etc.

2 files changed, 24 insertions, 5 deletions

diff --git a/utils/check-ppr/Main.hs b/utils/check-ppr/Main.hs
index c61b0e6d4c..8c937695cc 100644
--- a/utils/check-ppr/Main.hs
+++ b/utils/check-ppr/Main.hs
@@ -1,6 +1,7 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE RankNTypes #-}
 
+import Control.Monad (when)
 import Data.Data hiding (Fixity)
 import Data.List
 import Bag
@@ -20,15 +21,27 @@ import System.FilePath
 import qualified Data.ByteString as B
 import qualified Data.Map        as Map
 
-main::IO()
+usage :: String
+usage = unlines
+    [ "usage: check-ppr [--dump] (libdir) (file)"
+    , ""
+    , "where libdir is the GHC library directory (e.g. the output of"
+    , "ghc --print-libdir) and file is the file to parse."
+    , "The --dump flag causes check-ppr to produce .new and .old files"
+    , "containing dumps of the new and old ASTs in the event of a match"
+    , "failure."
+    ]
+
+main :: IO()
 main = do
   args <- getArgs
   case args of
-   [libdir,fileName] -> testOneFile libdir fileName
-   _ -> putStrLn "invoke with the libdir and a file to parse."
+   [libdir,fileName] -> testOneFile libdir fileName False
+   ["--dump", libdir,fileName] -> testOneFile libdir fileName True
+   _ -> putStrLn usage
 
-testOneFile :: FilePath -> String -> IO ()
-testOneFile libdir fileName = do
+testOneFile :: FilePath -> String -> Bool -> IO ()
+testOneFile libdir fileName dumpOldNew = do
        p <- parseOneFile libdir fileName
        let
          origAst = showAstData 0 (pm_parsed_source p)
@@ -56,6 +69,9 @@ testOneFile libdir fileName = do
            putStrLn origAst
            putStrLn "\n===================================\nNew\n\n"
            putStrLn newAstStr
+           when dumpOldNew $ do
+               writeFile (fileName <.> "old") origAst
+               writeFile (fileName <.> "new") newAstStr
            exitFailure
 
 
diff --git a/utils/check-ppr/README b/utils/check-ppr/README
index ac0eb55977..d31442a9e9 100644
--- a/utils/check-ppr/README
+++ b/utils/check-ppr/README
@@ -18,3 +18,6 @@ In a test Makefile
   $(CHECK_PPR) "`'$(TEST_HC)' $(TEST_HC_OPTS) --print-libdir | tr -d '\r'`" FileToParse.hs
 
 See examples in (REPO_HOME)/testsuite/tests/printer/Makefile
+
+If passed the --dump flag check-ppr will produce .new and .old files containing
+the ASTs before and after round-tripping to aid debugging.