Sorry,

I've just worked it out.

PortNumber is an instance of the class Enum so we can use

toEnum :: Int -> a

It's also an instance of the Integral class, which means you can just write integral constants of type PortNumber. eg. (80 :: PortNumber) is valid. Also, the universal integral conversion function 'fromIntegral' can be used for converting values of other integral types into PortNumbers (this is preferable to toEnum). Cheers, Simon