All arithmetic is performed modulo 2^n, where n is the number of bits in the type. One non-obvious consequence of this is that negate should not raise an error on negative arguments.
For coercing between any two integer types, use fromIntegral , which is specialized for all the common cases so should be fast enough. Coercing word types to and from integer types preserves representation, not sign.
It would be very natural to add a type Natural providing an unbounded size unsigned integer, just as Integer provides unbounded size signed integers. We do not do that yet since there is no demand for it.
The rules that hold for Enum instances over a bounded type such as Int (see the section of the Haskell report dealing with arithmetic sequences) also hold for the Enum instances over the various Word types defined here.
Right and left shifts by amounts greater than or equal to the width of the type result in a zero result. This is contrary to the behaviour in C, which is undefined; a common interpretation is to truncate the shift count to the width of the type, for example 1 << 32 == 1 in some C implementations.