- The Kit dependency manager was initially written in one weekend with little Haskell knowledge and resulted in bad code with IO everywhere and little usage of standard libraries. - Over time, the code was improved by removing IO usage, learning to leverage standard libraries and Hackage packages, cleaning up imports, and specifying invariants through QuickCheck properties and tighter types. - While an improvement, the code was still not considered "good" and could be further refined by fully removing IO from the monad stack and defining more precise invariants.

1. Writing Better
Haskell

2. TL;DR:You can start off
bad, and get better

3. Case Study: Kit

4. $ cabal install kit
Dependency manager for Obj-C code

5. Kit’s Origins
• 80% of the functionality needed produced
in one (alcohol fueled) weekend
• Very little Haskell knowledge
• No internet access, no guidance.

6. It’s bad code.

7. How bad?
• IO everywhere
• Very little usage of the standard library, or
any other libraries
• General ugliness

8. It could only get better.

9. Clearing out IO

10. Kit’s IO
• Producing Xcode projects - project files,
config files
• Reading Specs - a configuration file for a
Kit

11. Writing Files
createKitConfig :: Config -> IO ()

12. FS Actions
• data FSAction =
• FileCreate FilePath String |
• Symlink FilePath FilePath |
• InDir FilePath FSAction
• runAction :: FSAction -> IO ()

13. FSAction
createKitConfig :: Config -> FSAction
setupKitDirectory :: KitSpec ->
[FSAction]

14. Next: Cleaning IO
• Reader monad for specifying the
Environment
• Writer monad for pure logging
• State monad for pure repository
manipulation

15. Learning the
StdLib
[and hackage]

16. Unfolding
Dependencies
• treeDependencies :: Kit -> Tree Kit
• totalDependencies :: Kit -> [Kit]
• Order is actually important.

17. Unfolding
Dependencies
• Preserve order: lowest level deps first, keep
dep order listing from Spec file
• No duplicates

18. Unfolding
Dependencies
nub . concat . reverse . drop 1 . levels

19. Other learns:
• Globbing:
• Shelling out to ruby, vs
• the glob package
• CmdArgs:
• Pattern match on arrays of args, vs
• CmdArgs package

20. General Ugliness
Tidying up imports

21. Prefix headers
• Used in [Obj-]C applications to provide
global imports.
• Application-specific base library

22. Before
• import System.FilePath.Posix
• import System.Directory
• import Control.Applicative
• import Control.Monad
• In every damn file.

23. After
module Kit.Util(
module Kit.Util,
module Control.Applicative,
module Control.Monad,
module System.Directory,
module System.FilePath.Posix
) where

24. After
• import Kit.Util

25. So, is it good yet?

26. No.
• Plug through WriterT for Logging
• Remove IO from the Monad stack
• When IO is gone, define invariants in
quickcheck properties
• Learn how to specify those properties
through types