Getting started writing web applications in Haskell is pretty easy with FastCGI. Posted by Thomas Sutton on June 25, 2009

It’s pretty easy to get started with Haskell web-applications using the cgi and fastcgi packages, but a little bit of extra documentation never hurt anything. What follows is a brief run-down of getting a Haskell FastCGI program compiled and working under Apache with mod_fastcgi.

### Installing the software

There are two important pieces of software to install before we can build our application: Apache with mod_fastcgi, and the Haskell FastCGI library. You should install Apache as you normally do. This probably involves one of the following commands:

    sudo pacman -S apache # On Arch Linux
sudo aptitude install apache2 # On Debian, Ubuntu, etc.
sudo port install apache2 # On Mac OS X with Macports

#### Installing the FastCGI module

Next, you will need to install mod_fastcgi. This may be packaged for your system, in which case you can use pacman, aptitude, port, etc. as appropriate. If it is not, you’ll need to build it. Happily, this is dead easy:

Download and unpack the mod_fastcgi source from http://www.fastcgi.com/dist/. Edit Makefile.AP2 to point to your Apache root (the directory which contains build/). In my case (on Arch Linux), this was /usr/lib/httpd/. Compile and install it with make -f Makefile.AP2 && make -f Makefile.AP2 install.

Next, you’ll need to enable the module. The “correct” way to do this varies wildly from distribution to distribution. In particular, Debian-based systems like to spread this out over some three or four files.

Edit your Apache configuration (mine was in /etc/httpd/conf/httpd.conf) to load the module:

    LoadModule fastcgi_module modules/mod_fastcgi.so

There are a number of ways to configure the module, but the easiest is to add it as a handler for .fcgi files. Add the following snippet to httpd.conf:

    <IfModule fastcgi_module>
</IfModule>

You’ll still need to add the ExecCGI option to the directories which contain the .fcgi files to be run. I’m planning to use my ~/public_html/ directory, so I amended the <Directory> section in /etc/httpd/conf/extras/httpd-userdir.conf:

    Options +ExecCGI

Restart Apache and you’re good to go. If anything goes wrong, try looking at the mod_fastcgi documentation.

Installing the Haskell libraries should be a little more straightforward: just use Cabal.

1. cabal update
2. cabal install --global cgi
3. cabal install --global fastcgi

If your cabal does not know how to become root, you may need to prefix those cabal install commands with sudo (or, better yet, edit ~/.cabal/config to specify root-cmd: sudo).

### Building a test application

A simple Haskell FastCGI application looks like this (largely cribbed from Paul Brown’s blog):

    module Main ( main ) where

import Control.Concurrent
import Network.FastCGI

action :: CGI CGIResult
action = do
output $unlines [ "I am a FastCGI process!" , "Hear me roar!" , "" , show tid ] main :: IO () main = runFastCGIConcurrent' forkIO 10 action Copy and paste that code into a file (I’ll call it Main.hs) and compile it:  ghc -threaded --make -o ~/test.fcgi Main.hs You’ll now have a file ~/public_html/test.fcgi which you can call using curl. The first request might be a little slow, but subsequent requests should be much faster:  curl --include http://localhost/~$USER/test.fcgi