As discussed in the following thread:

http://www.haskell.org/pipermail/haskell-cafe/2008-August/046494.html

Add an option to look for modules using manifest files, rather than assuming that the filesystem contains a hierarchy of directories that exactly matches the module hierarchy.

This is desirable for the following reasons:

• It avoids an ugly and ad hoc dependency between two unrelated domains.
• In large and complex projects, it allows the project manager more flexibility in using directory structure to organize teams and manage complex version control setups.
• It avoids problems caused by mismatch between the filesystem and the module hierarchy, such as a non-hierarchical file system or incompatibilities in the way files are named.
• It allows for more than one module per file. This is helpful because the module system is the only way to achieve encapsulation in Haskell, so the system needs to be as lightweight and flexible as possible. It also makes it easier to share and distribute Haskell programs when it is most convenient to do so as a single file, such as literate Haskell via email.