Since the second week of this year’s GSOC is drawing to a close, I figured I’d take a minute to write a bit about my experience diving into Haskell development.

As someone relatively new to the Haskell world, I’ve had quite a bit to learn - fortunately, there’s pleny of documentation out there, and the IRC community is incredibly helpful as well (particularly #haskell, #hackage, and #haskell-beginners on Freenode).

However, the packaging and build tools themselves are in constant development, and working with cabal can be a bit tricky at first. In particular, there’s a lot of noise in forums and wikis concerning the best way to source your haskell packages - at this point, there are at several separate, viable ways to manage them:

• Install cabal from your distro’s package manager, use it to bootstrap cabal-install, then pull and build packages from hackage using cabal install <package_name>.

• Clone repos from git or darcs directly, and create binaries using cabal install in the project’s root directory (preferably using sandboxes), or

• Use one another tool to help streamline the process, such as the Nix package manager or Halcyon.

Personally, I’ve run into issues with some of these methods. I work across three different operating systems (Debian, Arch, and Windows), and coordinating equivalent packages can be tedious at best. Bootstrapping cabal-install can very quickly lead to the infamous “cabal hell”, in which it becomes difficult to keep track of exactly which packages are sourced as dependencies when you build something new. Cloning repos manually works fairly well, but in this case it’s difficult to install system-wide tools that rely on GHC such as HLint, ghc-mod, hdevtools, or really any other package that’s compiled against the GHC API.

For these reasons, I decided to combine approaches 3 and 4 to set up a reliable and easily reproducible dev environment – I rely on the package manager to provide an up-to-date version of GHC for building packages from source, and have a global config using halcyon that I can switch into with a few commands for doing dev work.

There are some pretty nice benefits to doing things this way - for example, I use the haskell-based xmonad for my window manager. With this setup, I can compile my configuration against the newest version of GHC, and not have to worry about one of its dependencies clobbering another project’s dependencies or having to roll back my global version of GHC to install older packages.

Overall, it’s proved so far to be a great way to keep dependencies cleanly separated, while still allowing multiple versions of ghc and cabal to be installed alongside each other. Setting things up is pretty straightforward, so here’s a quick rundown of what you can do to quickly get a dev environment rolling.

Install Halycon

In my case, it was easiest to start with a clean OS installation. Assuming you’re using a *nix variant, the steps should be roughly similar.

Start by installing halcyon – in my case, I did so as root to simplify things. You can find a tutorial over at https://halcyon.sh/tutorial/, but the key bit is to run:

and check that which ghc and which cabal both return paths in the /app/ directory.

From this point on, any user with access to the /app/ directory can call eval "\$( /app/halcyon/halcyon paths )” to jump into this environment - this command takes care of putting the correct versions of ghc and cabal at the front of your path, regardless of which versions you may have otherwise installed elsewhere.

You can then use halcyon install in place of cabal install to ensure that your packages are built against the particular versions of ghc and cabal you just installed.

Grab Some Dev Tools

In particular, if you’re doing development with these versions, you’ll want to build any tools that require executables using halcyon – here are a few examples to get you started:

Provides vim/emacs integration for type checking, linting, and showing compiler errors. Pairs really well with ghcmod-vim (https://github.com/eagletmt/ghcmod-vim) and syntastic (https://github.com/scrooloose/syntastic) for vim users.

A ctags alternative for Haskell projects. Use this to generate a .tags file for your project, and you can easily jump to function/type definitions using Ctrl-].

Uses hasktags to build your entire tags database with a single command

• codex update. What’s more, it also includes the tags of all of your project’s dependencies – so if, for example, you run cabal install inside of a sandbox, you can easily jump into the source code of other libraries and see function definitions, types, etc.

A cscope alternative for Haskell, which is kind of a reverse-direction ctags. After generating a database, you can press Ctrl-\ on a function definition to instantly find all of the places in your project that call that function. Paired with hasktags, jumping through a new codebase is a breeze.

Quickly parses a file and provides suggestions for style improvement. Can also be called using syntastic in vim (using :SyntasticCheck hlint).

A Haskell-specific search engine, lets you quickly look up function type signatures and definitions. Also has a neat feature that lets you search for functions by type signature – for example, searching for (a->b)->[a]->[b] brings up the map function and a few examples of how to use it. Super handy!

If you install any or all of these using halcyon install, their binaries will be placed in /app/bin/, and will be on your path any time you’re using halcyon’s environment.

Get Text Editor Integration

After trying several IDEs and plugins, I found that using vim with a few choice plugins netted me all of the features I really needed, and required the least troubleshooting.

The Necessities

If you’re just looking to get started as soon as possible, look no further than…

It has an automated setup, and provides a ton of great Haskell-specific tools right out of the box. (See the readme for the default key bindings.)

However, if you’re like me and already have a ridiculously long vimrc built up, take a look following repos for some useful plugins:

syntastic (https://github.com/scrooloose/syntastic)

Easily the plugin I use the most, and derive the most value from. If you have the ghc-mod binary on your path (which would be the case if you ran the eval command from earlier), then you can easily check your current file. With a few mappings like

, you can quickly get line markers and status messages for any errors. This also works within projects quite well, and will only recompile files that have changed since the last build (note: this can still be a bit slow for large projects).

ghcmod-vim (https://github.com/eagletmt/ghcmod-vim)

Can also perform type checking and linting, but one of its most useful features is the ability to display the type of an expression under the cursor by calling :GhcModType, or to pull up info about its definition with :GhcModInfo.

Mainly fixes up some extra syntax highlighting and indentation. Also offers a few neat features like looking up the word under the cursor on hoogle, and automatically adding qualified imports.

(https://github.com/enomsg/vim-haskellConcealPlus) This one’s purely aesthetic, and a great way to test exactly how well your terminal emulator support UTF-8! Uses vim conceals to show some Haskell operators and keywords as symbols (that is, it displays them as special symbols unless they are on your current line). This is particularly useful when you’re browing large swaths of code, and want to make things a bit more readable. Note: this does need a small hack not to be a complete eyesore. By default, vim adds a background highlight to every concealed character. You can clear this by throwing

General Vim Goodness

Here are some other useful plugins (not Haskell-specific) for jumping around large projects include:

Workflow

Finally, the good part! Everything’s easy from here on out. With everything installed, you can create a user and chown the /app/ directory. Then, you can go about your day-to-day business with your package manager’s version of ghc and cabal (just remember to run cabal sandbox init` before building/installing to keep things as clean as possible).

Then, whenever you’re working on a dev project, just call the eval statement mentioned up in the halcyon section, and all of the binaries you installed with halcyon will be on your path, all linked properly, and you’ll be using the same versions of ghc and cabal each and every time.

Generally, before I start working on a project, I run a function that looks something like this:

And there you have it! Hopefully this serves as a bit of help to anyone else getting started with Haskell. Please feel free to leave any questions or comments below!

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