Parse config files using parsec and generate parse errors on unhandled keys.
This is yet another entry in Haskell's enourmous collection of config-file parsing libraries. It lacks many of the bells and whistles of other config-file parsing libraries, such as hierarchical sections and on-the-fly reloading. On the other hand, it has a combination of features I was unable to find in other libraries:
Keys and values are parsed with configurable parsec parsers, resulting in flexible syntax and pretty error messages.
Custom parsers can be created with parsec to handle values of any type.
Keys that aren't explicitly handled result in parse errors.
If you don't need all of these features, there are probably better libraries out there for you. If you're free to use its idiosyncratic file format, the config-value library, in particular, is excelent.