I did send this mail a week ago or so to the haskell-libraries mailing list, but this was probably the wrong list. I have in the meantime tested with all cabbal files on haddock, and have fixed some issues. This patch adds a pretty printer for GenericPackageDescription. It fixes ticket http://hackage.haskell.org/trac/hackage/ticket/647. I added a new module Distribution.PackageDescription.PrettyPrint which exports writeGenericPackageDescription and showGenericPackageDescription. I use the field printers from Parse.hs, so that I had to add export statements for them in Distribution.PackageDescription.Parse. I decided to not use a class based approach because: 1. their is already the Text class, which has a parse I didn't want to write, and 2. and more important, I didn't want to use -XFlexibleInstances because I guess cabal should be portable. I have not removed the old printer code, although this may be a good idea (maybe mark it as deprecated first). Then it would make sense as well to rename writeGenericPackageDescription to writePackageDescription and showGenericPackageDescription to showPackageDescription, to be symmetric. I tested with the code from Appendix 1 and all the cabal files on hackage. The test succeds when writing and reading a package description results in an equal description. This is more rigorous then required, cause different GenericPackageDescr.. can have the same semantic, so for example the order of exe definitions is not relevant. But for our needs (GUI editor of cabal files for leksah), e.g. preserving of order is important and it will help to test the parser and printer to make this approach work. Furthermore I want to print out the cabal file as short as possible, e.g. not printing all empty fields like the old printer did. Testing the code I found the following problems: 1. Found up an obvious "age old" bug: hunk ./Distribution/PackageDescription/Parse.hs 359 - ghcProfOptions (\val binfo -> binfo{ghcSharedOptions=val}) + ghcSharedOptions (\val binfo -> binfo{ghcSharedOptions=val}) 2. The order of elements are reversed by the parser. E.g executables: (repos, flags, lib, exes, tests) <- getBody return (repos, flags, lib, exes ++ [(exename, flds)], tests) since the rest is parsed in the line before, the element needs to be prepended. so I changed to: (repos, flags, lib, exes, tests) <- getBody return (repos, flags, lib, (exename, flds): exes, tests) The same applies to test fields. The opposite order problem applies to storeXFields.., which is called in the order of processing, so I changed it to storeXFieldsPD (f@('x':'-':_),val) pkg = Just pkg{ customFieldsPD = (customFieldsPD pkg) ++ [(f,val)]} 3. The next problem is that we print nothing for empty fields , which works ok, except for compiler opts, where empty opts differ from not given opts, so I added this line to optsField in ParseUtils, which causes empty opts to be parsed, as if no opts are given: update f opts [] = [(f,opts)] 4. The next problem is that we loose final newlines with showFreeText. Astonishing enough the culpid is lines from the List module , which evals lines "a\n" to ["a"]. By the way even the claim that "'unlines' is an inverse operation to 'lines'" from the List module is not true, as unlines (lines "a") -> "a\n" (see Appendix 2). So I use my own unlines' now!. 5. This is connected to the CondTree structure. When I write a Cabl file like: ... library extensions: ForeignFunctionInterface if flag(use_xft) build-depends: X11-xft >= 0.2, utf8-string extensions: ForeignFunctionInterface ... the extension ForeignFunctionInterface in the conditional branch is superflous, as it is specified in any case, so the printer can simplify it to: ... library extensions: ForeignFunctionInterface if flag(use_xft) build-depends: X11-xft >= 0.2, utf8-string ... However the parse result for the two cabal files will differ. Now I gave up here, and added a variable simplifiedPrinting which disables the printer smartness for now. However, I hope someone can make the parser smarter in this respect. 6. If free text starts with a line with a dot, the dot must be in a new line. 7. Version tags have to be printed, which was not the case before (why?). 8. VersionRangeParens needs to be printed. 9. Compiler options have to be put in a canonical order after parsing, which is easy as CompilerFlavors do derive Ord. Kind regards Jürgen Nicklisch-Franken PS. I have added a remark about the new test suite syntax below. ------------------- Appendix1: Test code main = do allCabalFiles <- ... res <- mapM test allCabalFiles if and res then putStrLn "all test succeeded" else putStrLn $ "tests failed: " ++ show (length (filter (== False) res)) ++ " of: " ++ show (length res) test :: FilePath -> IO Bool test cabalFilePath = catch (do gpd <- readPackageDescription silent cabalFilePath writeGenericPackageDescription tempPath gpd gpd2 <- readPackageDescription silent tempPath if gpd == gpd2 then do putStr "." return True else do putStrLn $ "failed for " ++ cabalFilePath return False) (\e -> do putStrLn $ "runtime error " ++ show e ++ " for " ++ cabalFilePath return False) ------------------ Appendix2: lines and unlines revised: -- | 'unlines' is an inverse operation to 'lines'. -- It joins lines, after appending a terminating newline to each. unlines' :: [String] -> String unlines' [] = [] unlines' [x] = x unlines' (x:xs) = x ++ ('\n' : unlines' xs) -- | 'lines' breaks a string up into a list of strings at newline -- characters. The resulting strings do not contain newlines. lines' :: String -> [String] lines' [] = [""] lines' s = let (l, s') = break (== '\n') s in l : case s' of [] -> [] (_:s'') -> lines' s'' ---------------- Appendix 3: I find the test suite syntax problematic, for an exe I have to remeber the combination: test-suite foo type: exitcode-stdio-1.0 main-is: main.hs , and for a module: test-suite bar type: library-1.0 test-module: Bar So its the combination of exitcode-stdio- and main-is or library- and test-module which does the trick. Wouldn't it be easier to write something like: test-suite foo test-version: 1.0 main-is: main.hs , and for a module: test-suite bar test-version: 1.0 test-module: Bar