convertible provides a typeclass with a single function that is designed
to help convert between different types: numeric values, dates and times,
and the like. The conversions perform bounds checking and return a pure
Either value. This means that you need not remember which specific function
performs the conversion you desire. Also included in the package are
optional instances that provide conversion for various numeric and time
types, as well as utilities for writing your own instances. Finally, there
is a function that will raise an exception on bounds-checking violation,
or return a bare value otherwise, implemented in terms of the safer function
described above. Convertible is also used by HDBC 2.0 for handling
marshalling of data to and from databases. Convertible is backed by an
extensive test suite and passes tests on GHC and Hugs.

WWW: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/convertible