Dominic Steinitz
Is there a way of allowing someone to use AESKey in a type signature but not allow them to declare new instances?
I was going to suggest you export the class abstractly, that is, without its methods, so no-one could externally create an instance. But then I noticed that AESKey has no methods - it is just an alias for a combination of other classes - so that trick is insufficient. So, the other thing you could do is to introduce an indirection. module This (AESKey) where class (Bits a, Integral a) => AESKeyIndirection a -- un-exported class (AESKeyIndirection a) => AESKey a -- exported instance AESKeyIndirection Word128 instance AESKeyIndirection Word192 instance AESKeyIndirection Word256 instance AESKey Word128 instance AESKey Word192 instance AESKey Word256 Creating an instance of AESKey requires that you also create an instance of AESKeyIndirection. If only one of the names is available externally (exported), then instances can only be created internal to the module. Regards, Malcolm