Journey into Haskell, part 5

Haskell may be difficult to start out with, but once things start rolling, they roll fast. Yesterday (real world time, these blog entries are staggered) I had started the first lines of HackPorts, but now things are getting close to done for the first version. It's not that I've written much code, but that it was simple to integrate with other people's code. ## Borrowing all I can The first thing I wanted to do was avoid dealing with any of Hackage's data formats, so I cribbed everything I could from the `cabal-install` package. I actually imported the full source into HackPorts, ripped out its `List.hs` file, renamed it to my `Main.hs` file, and then began changing it from a function that prints out a list of available packages, to one that writes the data into properly formatted `Portfile` entries. The code does the following bits of work: 1. Talks to `cabal-install` and Cabal to get a list of all known packages on Hackage. 2. For every package, creates a directory named `haskell/$package`, and then writes information about that package into `haskell/$package/Portfile`. 3. As it does this, it fetches the current version's tarball over HTTP, and uses OpenSSL (directly, through FFI) to generate MD5, SHA1 and RIPEMD160 checksums of the tarball image. And voilá, a directory populated with 1136 Portfile entries. What's missing now is the external dependency mapping. As a stub, I have them all depending on `port:ghc`, but I think there's sufficient information in the Cabal package info to figure out what the right dependencies should be, both among the Hackage packages themselves and against any external libraries (like OpenSSL). ## What I learned As for my Haskell education, I learned about using Haskell's very nice FFI mechanism, and had a lot more experience using the IO Monad. An example of using FFI to call out to OpenSSL: {-# OPTIONS -#include "openssl/md5.h" #-} foreign import ccall "openssl/md5.h MD5" c_md5 :: Ptr CChar -> CULong -> Ptr CChar -> IO (Ptr Word8) I now have access to a `c_md5` function, which go directly over to the C library to do its work. Not too shabby! As for the IO Monad, here is the `main` function for Hackports: main :: IO () main = do createDirectoryIfMissing True "haskell" pkgs

Syndicated 2009-03-24 13:00:00 (Updated 2009-03-23 05:28:22) from Lost in Technopolis