Download the Mac OS X installer here:
Haskell Platform 2012.4.0.0 for Mac OS X, 32 bit (10.6 or later)
Haskell Platform 2012.4.0.0 for Mac OS X, 64 bit (10.6 or later)
Only one architecture (32-bit or 64-bit) can be installed at a time.
The command line development tools are required prior to installation.
If you have /usr/bin/gcc available in a shell, you should be good to go. If not:
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.
On OS 10.7, 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, 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 10.7 option above.
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).
MacPorts, brew, and other native libraries:
Install the version (32-bit or 64-bit) that matches the architecture of the libraries you intend to work with.
By default, MacPorts, brew and most libraries only build 64-bit versions on 64-bit capable systems. Hence you need to pick the 64-bit version of the platform to use it with these libraries.
You might be able to install libraries "universal", which will make them work with either version of the platform, but not all libraries build this this way. Try the commands port install libfoo +universal or brew install libfoo --universal
For Mac OS X 10.5 please use the 2010.2.0.0 installer: