Getting Started with Haskell
I’ve been having a lot of fun learning Haskell these past few months, but getting started isn’t quite as straight-forward as it could be. I had the good fortune to work at the right place at the right time and was able to take Bryan O’Sullivan’s Haskell class at Facebook, but it’s definitely possible to get started on your own. While you can play a bit with Haskell at Try Haskell you’ll eventually want to get GHC installed on your own machine.
- Install the Haskell Platform (GHC)
- Set up Cabal
- Install Cabal-dev (sandbox build tool)
- Install ghc-mod (better Emacs/Vim support)
- How to install tools with cabal-dev
- Configure GHCi
- Hackage is fragile, but there are (unofficial) mirrors
- Starting a project (with cabal-dev)
- GHCi basics
- Recommended Reading
Install the Haskell Platform (GHC)
The Haskell Platform is the Glasgow Haskell Compiler (GHC) including the “batteries included” standard library. GHC not the only Haskell compiler, but this is the one that you want to learn. Another implementation of note is Hugs, which is more for teaching than for production code.
These instructions are written for Mac OS X 10.8 using Homebrew (and a recent version of Xcode), but it should be easy to figure out how to do it on other platforms starting from Haskell Platform. The current version of Haskell Platform at this time is 2012.4.0.0.
$ brew install haskell-platform
Set up Cabal
Cabal is Haskell’s Common Architecture for Building Applications and Libraries. In combination with Hackage it is similar in purpose to tools such as CPAN for Perl, pip for Python, or gem for Ruby. You’ll probably be disappointed, but it’s not that bad.
When you end up installing packages with cabal, it will install them
~/.cabal/ and the scripts will go into
should go ahead and add this to your PATH environment
variable now. Something like this will suffice, but it depends on how
you like to set up your shell profile:
$ echo 'export PATH=$HOME/.cabal/bin:$PATH' >> ~/.bashrc
Before doing anything else with cabal, you’ll need to bootstrap the list of available packages. You’ll want to run this occasionally, particularly before installing or upgrading new packages.
$ cabal update
At this point you’ll have a
~/.cabal/config that doesn’t have
library-profiling turned on. You’ll likely want this later, and if you don’t
do it now then you’ll have to rebuild everything later. To turn it on edit
~/.cabal/config and change
-- library-profiling: False to
$ for f in ~/.cabal/config; do \ cp $f $f.old && \ sed -E 's/(-- )?(library-profiling: )False/\2True/' < $f.old > $f; \ done
Before installing anything else, you’ll need to install the Cabal installer:
$ cabal install cabal-install
Install ghc-mod (better Emacs/Vim support)
ghc-mod is what you want to install to integrate GHC with Emacs or Vim. You might also be able to use Sublime Text 2 and ghc-mod via SublimeHaskell. I’ve only tried the Emacs integration so far. Vim users may want to try hdevtools as it’s much faster and just as accurate (see kamatsu’s comment).
$ cabal install ghc-mod
You’ll obviously have to configure it for your Emacs, and I’ll leave that up to you (my current ~/.emacs.d for reference).
Install Cabal-dev (sandbox build tool)
Cabal-dev is a tool that helps you sandbox installation of Haskell software. It is similar in purpose to virtualenv for Python or rvm for Ruby, but the usage is quite a bit different. This is the tool that will save you from “Cabal Hell”, where you can’t install a package because some other package you have installed has conflicting dependencies.
cabal-dev instead of just
cabal to build stuff whenever possible. The major trade-off is that you will spend (a lot) more time compiling packages that you already have installed somewhere else (and waste some disk), but this is almost certainly a fair trade.
$ cabal install cabal-dev
There’s some work in progress for adding Sandboxed Builds and Isolated Environments support to cabal-install, so the cabal-dev material here will likely bit rot in a few months (years?).
How to install tools with cabal-dev
If you want to try out a tool, but don’t want to pollute your Haskell installation, you can just use cabal-dev! By default, cabal-dev’s sandbox is
./cabal-dev, but you can put it anywhere. In this example I’ll install darcs 2.8.2 (a distributed version control system written in Haskell) into
/usr/local/Cellar/darcs/2.8.2 and have Homebrew create the symlinks for me. On other platforms you might want to use your own directory structure and manage your
$ cabal-dev install -s /usr/local/Cellar/darcs/2.8.2 darcs-2.8.2 $ brew link --overwrite darcs
Bam! Now darcs is on your
PATH and you don’t have to worry about version conflicts. Well, you do sadly still run into them, just not as much. Specifically, cabal-dev installs packages in such a way that they are all ‘top-level’ in a given sandbox. This means that if two packages have common dependencies (VERY common), then they’ll stomp on each other’s symlinks to things like license files and documentation of the dependencies. It’s mostly harmless to use
--overwrite in this way, but you might want to check with
--overwrite --dry-run first. Annoying, but probably won’t ruin your day.
If you want to see what versions of a darcs are available, use
cabal info darcs and look for the
Versions available: section.
Other fun Haskell tools to try (in no particular order):
- pandoc - the swiss-army knife of markup format converters (markdown, reStructuredText, org-mode, LaTeX, etc.)
- gitit - a wiki backed by a git, darcs or mercurical filestore
- pronk - a HTTP load testing tool, like ab or httperf, only more modern and simpler to deal with
For packages like pronk that aren’t currently in Hackage, a cabal-dev installation will look more like this:
$ git clone https://github.com/bos/pronk.git /tmp/pronk-src && \ (cd /tmp/pronk-src; \ cabal-dev install -s /usr/local/Cellar/pronk/$(git rev-parse --short HEAD)) && \ rm -rf /tmp/pronk-src
ghci is the GHC interactive interpreter (REPL, similar to typing
irb in a shell). For real documentation, see the GHC Users Guide (Chapter 2. Using GHCi) . You’ll be spending a lot of time there playing with your code, you probably want to set up a shorter prompt. It starts off looking like this:
Once you start importing modules the prompt keeps getting longer and you really just don’t need that in your life.
Prelude> :m + Data.List Prelude Data.List> :m + Data.Maybe Prelude Data.List Data.Maybe>
The configuration file for this is the .ghci file. I use a very simple ASCII prompt, some people like to make theirs look like
echo ':set prompt "h> "' >> ~/.ghci
You could also issue the
:set prompt "h> " command each time you use GHCi, but that gets old.
$ ghci h> putStrLn "Hello World!" Hello World! h>
Hackage is fragile, but there are (unofficial) mirrors
Sadly, Hackage isn’t currently the pinnacle of reliability. I don’t know what the problem is, but hopefully they do something about it soon. There is a workaround (see also Working around Hackage Outages), you can just use the repo from hdiff at hdiff.luite.com or from hackage.csc.stanford.edu.
Change this line in
<pre class="light plain literal-block">
To something like this:
-- TODO When hackage is back up, set back to hackage.haskell.org! -- remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive remote-repo: hdiff.luite.com:http://hdiff.luite.com/packages/archive -- remote-repo: hackage.csc.stanford.edu:http://hackage.scs.stanford.edu/packages/archive
After you’ve changed your remote-repo setting, you’ll need to update the package list
$ cabal update
Don’t forget to change it back later!
Starting a project (with cabal-dev)
You’d figure this out eventually, but a quick way to start out a project is to just go ahead and start off with cabal-dev. Here’s how you would do that for a trivial program.
For your own projects, you may want to remove the
-n option and let cabal
ask you which options you want to choose. The
-n option uses all the
defaults without any prompting.
$ mkdir -p ~/src/hs-hello-world $ cd ~/src/hs-hello-world $ touch LICENSE $ cabal init -n --is-executable
This will generate a
hs-hello-world.cabal. The next step is to
main-is: line so it knows what source file to build your
executable from. The end result should look something like this:
-- Initial hs-hello-world.cabal generated by cabal init. For further -- documentation, see http://haskell.org/cabal/users-guide/ name: hs-hello-world version: 0.1.0.0 -- synopsis: -- description: license: AllRightsReserved license-file: LICENSE -- author: -- maintainer: -- copyright: -- category: build-type: Simple cabal-version: >=1.8 executable hs-hello-world main-is: HelloWorld.hs -- other-modules: build-depends: base ==4.5.*
Then create a HelloWorld.hs, maybe something that looks like this:
main :: IO () main = putStrLn "Hello, world!"
You can build and “install” it into a local sandbox like this:
$ cabal-dev install Resolving dependencies... Configuring hs-hello-world-0.1.0.0... Building hs-hello-world-0.1.0.0... Preprocessing executable 'hs-hello-world' for hs-hello-world-0.1.0.0... Installing executable(s) in /Users/bob/src/hs-hello-world/cabal-dev//bin Installed hs-hello-world-0.1.0.0 $ ./cabal-dev/bin/hs-hello-world Hello, world!
The executable is large for what it does, but it’s also statically linked. You can copy that file over to any other machine with the same OS and architecture and it’ll Just Work.
You might save a tiny bit of time by skipping the install step:
$ cabal-dev configure Resolving dependencies... Configuring hs-hello-world-0.1.0.0... $ cabal-dev build Building hs-hello-world-0.1.0.0... Preprocessing executable 'hs-hello-world' for hs-hello-world-0.1.0.0... [1 of 1] Compiling Main ( HelloWorld.hs, dist/build/hs-hello-world/hs-hello-world-tmp/Main.o ) Linking dist/build/hs-hello-world/hs-hello-world ... $ ./dist/build/hs-hello-world/hs-hello-world Hello, world!
Since this project has no dependencies that you want to install locally, you can take some shortcuts.
Run it interpreted, no compilation step needed:
$ runghc HelloWorld.hs Hello, world!
$ ghci GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :load HelloWorld [1 of 1] Compiling Main ( HelloWorld.hs, interpreted ) Ok, modules loaded: Main. *Main> main Hello, world!
And you can build it without cabal-dev (or cabal) at all:
$ runghc Setup.hs configure Configuring hs-hello-world-0.1.0.0... $ runghc Setup.hs build Building hs-hello-world-0.1.0.0... Preprocessing executable 'hs-hello-world' for hs-hello-world-0.1.0.0... [1 of 1] Compiling Main ( HelloWorld.hs, dist/build/hs-hello-world/hs-hello-world-tmp/Main.o ) Linking dist/build/hs-hello-world/hs-hello-world ...
But for a more complicated project, you can use
cabal-dev ghci (after
cabal-dev configure && cabal-dev build).
Note that it loads your executable’s source into the interpreter automatically:
$ cabal-dev ghci on the commandline: Warning: -O conflicts with --interactive; -O ignored. GHCi, version 7.4.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Ok, modules loaded: Main. h> main Hello, world!
Some essential GHCi tricks you’ll want to know, you’ll find more in Chapter 2. Using GHCi.
:t shows the type of an expression
h> :t main main :: IO () h> :t map map :: (a -> b) -> [a] -> [b] h> :t map (+1) map (+1) :: Num b => [b] -> [b]
:i shows information about a name (function, typeclass, type, …)
h> :i Num class Num a where (+) :: a -> a -> a (*) :: a -> a -> a (-) :: a -> a -> a negate :: a -> a abs :: a -> a signum :: a -> a fromInteger :: Integer -> a -- Defined in `GHC.Num' instance Num Integer -- Defined in `GHC.Num' instance Num Int -- Defined in `GHC.Num' instance Num Float -- Defined in `GHC.Float' instance Num Double -- Defined in `GHC.Float' h> :info map map :: (a -> b) -> [a] -> [b] -- Defined in `GHC.Base' h> :info Int data Int = ghc-prim:GHC.Types.I# ghc-prim:GHC.Prim.Int# -- Defined in `ghc-prim:GHC.Types' instance Bounded Int -- Defined in `GHC.Enum' instance Enum Int -- Defined in `GHC.Enum' instance Eq Int -- Defined in `ghc-prim:GHC.Classes' instance Integral Int -- Defined in `GHC.Real' instance Num Int -- Defined in `GHC.Num' instance Ord Int -- Defined in `ghc-prim:GHC.Classes' instance Read Int -- Defined in `GHC.Read' instance Real Int -- Defined in `GHC.Real' instance Show Int -- Defined in `GHC.Show'
:m add a module to the scope
h> :m + Data.List h> sort [10,9..1] [1,2,3,4,5,6,7,8,9,10]
:l load a module,
:r to reload
h> :! echo 'hello = print "hello"' > Hello.hs h> :l Hello [1 of 1] Compiling Main ( Hello.hs, interpreted ) Ok, modules loaded: Main. h> hello "hello" h> :! echo 'hello = print "HELLO"' > Hello.hs h> :r [1 of 1] Compiling Main ( Hello.hs, interpreted ) Ok, modules loaded: Main. h> hello "HELLO"
These are the books and sites that I found particularly useful while trying to learn Haskell myself.
Learn You a Haskell for Great Good!: A Beginner’s Guide
I found this one to be a great starting point, I would recommend that you read it first. It doesn’t go so deep that you feel like you REALLY understand GHC works, but I felt pretty comfortable reading and writing Haskell after getting through this.
Real World Haskell
This book is massive in size and scope, but is still very accessible for beginners. It’ll teach you how to do Real World things with Haskell: writing tests, profiling, IO, concurrency, etc. I’m still working through this one, but it’s a must-read.
- CS240h: Functional Systems in Haskell was a Haskell class at Stanford taught by David Mazières and Bryan O’Sullivan. It’s similar (but larger in scope) to the class I took at Facebook. The lecture notes and syllabus here are fantastic, go through them!
- Real World Haskell
- Learn You a Haskell
- haskell.org is a great entry point, you can find all of the links in this list and MANY more from there. Plan to spend a lot of time browsing this wiki!
- H-99 has a bunch of little problems to work on, much like the Euler project. These should be pretty straightforward to do after reading LYAH.
- Typeclassopedia is a great resource for learning about many of the prominent typeclasses in the Haskell Platform
- Hoogle is a Haskell API search engine that supports searching by type signature! I spend a lot of time with this one
- Hayoo! is another Haskell API search engine, worth a shot if you can’t find what you’re looking for on Hoogle
- HWN is a weekly Haskell newsletter that gives you the highlights of the mailing lists, stackoverflow, reddit, etc.
- Haskell :: Reddit is the subreddit for Haskell
- stackoverflow - haskell The questions tagged Haskell on stackoverflow are often worth reading (although to be honest I usually end up here from HWN)
- C9 Lectures: FP Fundamentals 13 lectures on Functional Programming Fundamentals (in Haskell) by Dr. Erik Meijer (I haven’t watched them yet, but they were suggested by Adam Breen).
haskell on Freenode is where you’ll find a few hundred people
interested in Haskell at any given time. Great place for help.
I plan to try and keep this current based on suggestions. Let me know if I’m missing anything of note! I’m not trying to be comprehensive, I think the Haskell wiki does a far better job of that. These are just intended to be highlights.