Make parsed AST dump output lazily

Previously, `showAstData` produced a `String`. That `String` would
then be converted to a `Doc` using `text` to implement
`-ddump-parsed-ast`. But rendering `text` calculates the length
of the `String` before doing anything else. Since the AST can be
very large, this was bad: the whole dump string (potentially hundreds
of millions of `Char`s) was accumulated in memory.

Now, `showAstData` produces a `Doc` directly, which seems to work
a lot better. As an extra bonus, the code is simpler and cleaner.
The formatting has changed a bit, as the previous ad hoc approach
didn't really match the pretty printer too well. If someone cares
enough to request adjustments, we can surely make them.

Reviewers: austin, bgamari, mpickering, alanz

Reviewed By: bgamari

Subscribers: mpickering, rwbarton, thomie

GHC Trac Issues: #14161

Differential Revision: https://phabricator.haskell.org/D3894

1 files changed, 5 insertions, 2 deletions

diff --git a/utils/check-ppr/Main.hs b/utils/check-ppr/Main.hs
index 2fd44b2be0..a5aeee2f1d 100644
--- a/utils/check-ppr/Main.hs
+++ b/utils/check-ppr/Main.hs
@@ -31,7 +31,8 @@ testOneFile :: FilePath -> String -> IO ()
 testOneFile libdir fileName = do
        p <- parseOneFile libdir fileName
        let
-         origAst = showAstData BlankSrcSpan (pm_parsed_source p)
+         origAst = showSDoc unsafeGlobalDynFlags
+                     $ showAstData BlankSrcSpan (pm_parsed_source p)
          pped    = pragmas ++ "\n" ++ pp (pm_parsed_source p)
          anns    = pm_annotations p
          pragmas = getPragmas anns
@@ -45,7 +46,9 @@ testOneFile libdir fileName = do
 
        p' <- parseOneFile libdir newFile
 
-       let newAstStr = showAstData BlankSrcSpan (pm_parsed_source p')
+       let newAstStr :: String
+           newAstStr = showSDoc unsafeGlobalDynFlags
+                         $ showAstData BlankSrcSpan (pm_parsed_source p')
        writeFile newAstFile newAstStr
 
        if origAst == newAstStr