How to maintain Haskell Packages in Nix ======================================= ## Overview over the tool-chain There are two utilities, `cabal2nix` and `hackage4nix`, that automate maintenance to a large extend. We intend to merge them into one program, eventually, but the necessary re-factoring hasn't been done yet since this is not a high priority. ### Cabal2nix `cabal2nix` converts a single Cabal file into a single Nix build expression. For example: $ cabal2nix cabal://yesod-0.9.1 { cabal, attoparsecText, blazeBuilder, blazeHtml, hamlet, httpTypes , monadControl, parsec, shakespeareCss, shakespeareJs, text, time , transformers, unixCompat, wai, waiExtra, warp, yesodAuth , yesodCore, yesodForm, yesodJson, yesodPersistent }: cabal.mkDerivation (self: { pname = "yesod"; version = "0.9.1"; sha256 = "1ag3lca75lrriycbscspb5yyishacgxjx0rybc3x4z1dqnkn1r71"; isLibrary = true; isExecutable = true; buildDepends = [ attoparsecText blazeBuilder blazeHtml hamlet httpTypes monadControl parsec shakespeareCss shakespeareJs text time transformers unixCompat wai waiExtra warp yesodAuth yesodCore yesodForm yesodJson yesodPersistent ]; meta = { homepage = "http://www.yesodweb.com/"; description = "Creation of type-safe, RESTful web applications"; license = self.stdenv.lib.licenses.bsd3; platforms = self.ghc.meta.platforms; }; }) Cabal files can be referred to using the magic URL `cabal://NAME-VERSION`, which will automatically download the file from Hackage. Alternatively, a direct `http://host/path/pkg.cabal` URL can be provided, as well as a `file:///local/path/pkg.cabal` URI that doesn't depend on network access. Run the utility with `--help` to see the complete list of supported command line flags. To add a new package to Nix, the following steps need to be performed: $ cd $NIXPKGS_ALL/pkgs/development/libraries/haskell $ mkdir foo $ cabal2nix cabal://foo-1.0 >foo/default.nix Then add an appropriate attribute to `pkgs/top-level/haskell-packages.nix`, for example: foo = callPackage ../development/libraries/haskell/foo {}; ### Hackage4nix The `hackage4nix` utility re-generates *all* expressions found in the `nixpkgs` database in place. This is useful to ensure that all packages have been generated with a recent version of the tool-chain. Furthermore, `hackage4nix` adds default settings for the `meta.maintainers` and `meta.platforms` attribute if these aren't configured yet. Generally speaking, running $ hackage4nix $NIXPKGS_ALL should be a *no-op* --- i.e. no files should change! If there are changes, these indicate that a file has been modified manually, and then these changes must be investigated to find out what is going on. Last but not least, `hackage4nix` generates a list of all updates available from Hackage. (Run `cabal update` to make sure that your local copy of the Hackage database is up-to-date!) For example: $ hackage4nix $NIXPKGS_ALL The following updates are available: WebBits-Html-1.0.1: cabal2nix cabal://WebBits-Html-1.0.2 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/WebBits-Html/default.nix happstack-server-6.1.6: cabal2nix cabal://happstack-server-6.2.1 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/happstack/happstack-server.nix cabal2nix cabal://happstack-server-6.2.2 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/happstack/happstack-server.nix primitive-0.3.1: cabal2nix cabal://primitive-0.4 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/primitive/default.nix repa-2.1.1.5: cabal2nix cabal://repa-2.1.1.6 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/repa/default.nix unix-compat-0.2.2.1: cabal2nix cabal://unix-compat-0.3 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/unix-compat/default.nix vector-0.7.1: cabal2nix cabal://vector-0.8 >$NIXPKGS_ALL/pkgs/development/libraries/haskell/vector/default.nix These updates can be performed automatically by running the `cabal2nix` command given by `hackage4nix`. If there is more than one possible update, then all of them will be shown. Note, however, that some updates break compilation of other packages, because they depend on very specific versions of their build inputs, so please be careful when performing updates! ## Current State of Affairs The tool-chain is stable. As of today, 2011-08-30, almost all Haskell packages available in Nix have been generated automatically from their Cabal files. The exceptions are: * "Bad packages" that cannot be generated automatically because there is no proper Cabal file for them, or because their builds are so complicated that the `cabal.nix` build doesn't suffice to do the trick. The list of bad packages is hard-coded into [`hackage4nix.hs`](http://github.com/NixOS/cabal2nix/blob/master/src/Hackage4Nix.hs), in the functions `badPackagePaths` and `patchedPackages`. Any package list in there is going to be ignored by `hackage4nix`. * "Patched packages" cannot be generated automatically, because their build instructions need patches that `cabal2nix` doesn't (yet) know about. There is a hard-coded list of patched packages in [`hackage4nix`](http://github.com/NixOS/cabal2nix/blob/master/src/Hackage4Nix.hs). Furthermore, all build expressions that define any of the attributes (pre|post)Configure (pre|post)Install patchPhase patches are considered "patched". That list of attributes is hard-coded in [`hackage4nix.hs`](http://github.com/NixOS/cabal2nix/blob/master/src/Hackage4Nix.hs) in the function `regenerateDerivation`. The complete list of Haskell packages available in Nix is generated by the tool [`package-list`](http://github.com/peti/package-list), and published at . Hackage picks it up from there and generates links on each package's homepage to the corresponding page in Hydra automatically. See [ticket 875](http://hackage.haskell.org/trac/hackage/ticket/875) for further details.