Download the Mac OS X installer here:
Haskell Platform 2014.2.0.0 for Mac OS X, 64bit (10.6 or later)
activate-hscommand that can switch between multiple installed versions of the platform
~/.cabal/configfile the first time you run it. Please pay attention to its output. If you have a custom config file, you'll want to update it, as Cabal's defaults have changed.
The command line development tools are required prior to installation.
If you have /usr/bin/ld available in a shell, you should be good to go. If not:
On OS 10.7 or later, choose one of the following:
Download and install Command Line Tools for Xcode. Despite the name, you don't need Xcode installed!
After installing Xcode (4.3 or later), choose Preferences, then pick the Downloads panel. There you can download and install the Command Line Tools as an optional component. This installs the same package as listed in the above option.
If you have Xcode prior to 4.3, depending on how you installed it, you may already have the command line tools. If not, consider the first option above.
On OS 10.6, 32-bit, use Xcode 3.2 or later: Choose the "Customize…" button during installation and choose UNIX Development. 64-bit version requires Xcode 4.1 or later.
The command line tools and/or Xcode can be downloaded for free from Apple's developer website, you do need to register as a developer (also free).
For Mac OS X 10.5 please use the 2010.2.0.0 installer: