Bob Ippolito (@etrepum) on Haskell, Python, Erlang, JavaScript, etc.
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Getting Started with Haskell

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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)

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 to ~/.cabal/ and the scripts will go into ~/.cabal/bin/. You 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 library-profiling: True.

$ 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.

Use 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 PATH instead.

$ 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

Configure GHCi

ghci is the GHC interactive interpreter (REPL, similar to typing python or 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:

Prelude>

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 λ>.

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 ~/.cabal/config: <pre class="light plain literal-block"> remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive </pre>

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 Setup.hs and hs-hello-world.cabal. The next step is to change the main-is: line so it knows what source file to build your executable from. The end result should look something like this:

hs-hello-world.cabal

-- 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:

HelloWorld.hs

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!

GHCi Basics

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"

Recommended Reading

These are the books and sites that I found particularly useful while trying to learn Haskell myself.

Books

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.


Sites

  • 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).

IRC

  • haskell on Freenode is where you’ll find a few hundred people

    interested in Haskell at any given time. Great place for help.

Suggestions?

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.

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