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 ghc-mod (better Emacs/Vim support)
- How to install tools with cabal sandbox
- Configure GHCi
- Hackage is fragile, but there are (unofficial) mirrors
- Starting a project (with cabal sandbox)
- 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.
$ brew install ghc cabal-install
At time of this writing, the
ghc package is at version 7.8.4, and
cabal-install at 126.96.36.199. The Haskell Platform installer for all platforms
is version 2014.2.0.0.
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
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). There are numerous Vim plugins you might evaluate, a one-shot starter setup using ghc-mod is haskell-vim-now or you may look at its constituent parts to kickstart a do-it-yourself configuration.
How to install tools with
cabal sandbox is a tool that helps you isolate project dependencies, and can also be used to sandbox installation of Haskell software. It is similar in purpose to virtualenv for Python or rvm for Ruby, but the usage is 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.
If you want to try out a tool, but don’t want to pollute your Haskell installation, you can just use
cabal sandbox! By default
cabal sandbox creates a
.cabal-sandbox directory under the current working directory, for use in each of your own projects, but you can create sandboxes anywhere. In this example I’ll install darcs 2.8.5 (a distributed version control system written in Haskell) into
/usr/local/Cellar/darcs/2.8.5 and have Homebrew create the symlinks for me. On other platforms you might want to use your own directory structure, such as
/opt/local, and manage your
$ mkdir -p /usr/local/Cellar/darcs/2.8.5 $ cd !$ $ cabal sandbox init --sandbox . $ cabal install darcs-2.8.5 $ 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 sandbox installs packages in such a way that they are all ‘top-level’ in a given sandbox (and this is why we specify
--sandbox . for
init above, to override placement into a child
.cabal-sandbox directory). 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 sandbox installation will look more like this:
$ git clone https://github.com/bos/pronk.git /tmp/pronk-src && pushd /tmp/pronk-src $ version=$(git rev-parse --short HEAD) $ export CABAL_SANDBOX_CONFIG=/usr/local/Cellar/pronk/$version/cabal.sandbox.config $ cabal sandbox init --sandbox=/usr/local/Cellar/pronk/$version $ cabal install $ popd && rm -rf /tmp/pronk-src && unset CABAL_SANDBOX_CONFIG
cabal sandbox instead of just
cabal install 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.
We will have a further look at using Cabal sandboxes for dependency isolation of your own projects further below.
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
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
You’d figure this out eventually, but a quick way to start out a project is
cabal init, and you’ll save yourself a world of dependency hell if you
cabal sandbox from day one. Here’s how you would do that for a trivial
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. We specify a license to avoid build complaints
$ mkdir -p ~/src/hs-hello-world $ cd ~/src/hs-hello-world $ cabal init -n --is-executable --license=MIT
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: MIT license-file: LICENSE -- author: -- maintainer: -- copyright: -- category: build-type: Simple cabal-version: >=1.10 executable hs-hello-world main-is: HelloWorld.hs -- other-modules: build-depends: base >=4.7 && <4.8
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 sandbox init Writing a default package environment file to /Users/bob/src/hs-hello-world/cabal.sandbox.config Creating a new sandbox at /Users/bob/src/hs-hello-world/.cabal-sandbox $ cabal install Resolving dependencies... Notice: installing into a sandbox located at /Users/bob/src/hs-hello-world/.cabal-sandbox Configuring hs-hello-world-0.1.0.0... Building hs-hello-world-0.1.0.0... Installed hs-hello-world-0.1.0.0 $ ./.cabal-sandbox/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.
This shows explicitly where the sandbox lives and that our executable is
installed into it just like library dependencies would be. But there is a Cabal
convenience command for building your project and running an executable—
$ cabal configure Resolving dependencies... Configuring hs-hello-world-0.1.0.0... $ cabal run 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 ... Running hs-hello-world... Hello, world!
Here we see that a build (which you could run separately with
produces artifacts in
./dist/build. For a project with only a single
executable, a bare
cabal run will execute it. An explicit
hs-hello-world also works, and bash completion for Cabal is available that
even completes from the target names in the project
.cabal file (installed
automatically with Homebrew if you have the
installed). This might also save a tiny bit of time by skipping extra
Since this project has no dependencies that you want to install locally, you can take some shortcuts that you might use for testing quick one-off Haskell programs.
Run it interpreted, no compilation step needed:
$ runhaskell HelloWorld.hs Hello, world!
$ ghci GHCi, version 7.8.4: 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 at all:
$ cabal clean cleaning... $ runhaskell Setup.hs configure Configuring hs-hello-world-0.1.0.0... $ runhaskell 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 ...
The above GHCi example shows how you can load a bare module anywhere into the
REPL, but normally in a Cabal project you’ll want to run
cabal repl instead.
This starts GHCi with your project build environment set up and thus your
sandbox dependencies also available for import in the REPL. Note that our main
module is available automatically:
$ cabal repl Preprocessing executable 'hs-hello-world' for hs-hello-world-0.1.0.0... GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( HelloWorld.hs, interpreted ) Ok, modules loaded: Main. h> main Hello, world!
Hackage Dependencies and Freezing
Our trivial example project doesn’t have any library dependencies from Hackage.
ansi-terminal to spruce up our sophisticated program, and give us
an example to work with. Modify the
build-depends section of
hs-hello-world.cabal as follows:
build-depends: base >=4.7 && <4.8, ansi-terminal == 0.6.*
Now, add some bling to
import System.Console.ANSI main :: IO () main = do putStr "Hello, " setSGR [ SetColor Foreground Vivid Green ] putStrLn "world!"
A configure will remind us the dependency is missing:
$ cabal configure Resolving dependencies... Configuring hs-hello-world-0.1.0.0... cabal: At least the following dependencies are missing: ansi-terminal ==0.6.*
So simply install it:
$ cabal install Resolving dependencies... Notice: installing into a sandbox located at /Users/bob/src/hs-hello-world/.cabal-sandbox Configuring ansi-terminal-0.6.2.1... Building ansi-terminal-0.6.2.1... Installed ansi-terminal-0.6.2.1 Configuring hs-hello-world-0.1.0.0... Building hs-hello-world-0.1.0.0... Installed hs-hello-world-0.1.0.0
cabal run should reveal a brighter world!
We’ve declared a fuzzy dependency on
ansi-terminal. If this were an open
source library, this would be fine—project contributers or CI might install a
new 0.6.x version, and if something breaks you’ll know it needs to be fixed to
satisfy the declared compatibility. In private production projects of course,
you probably want strict build consistency.
cabal freeze is here to help. Run
cabal freeze and you’ll find a new
cabal.config file in the project directory, which will look something like
constraints: ansi-terminal ==0.6.2.1, array ==0.5.0.0, base ==188.8.131.52, bytestring ==0.10.4.0, deepseq ==184.108.40.206, ghc-prim ==0.3.1.0, integer-gmp ==0.5.1.0, old-locale ==220.127.116.11, rts ==1.0, time ==1.4.2, unix ==18.104.22.168
This file locks down the exact versions of dependencies (including transitive) installed in your development sandbox. Check this file into source control, and you will establish build consistency throughout your team and build pipeline using Cabal.
Refer to the Cabal User’s
Guide for more on using
.cabal build definitions—like adding tests to you builds!
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.