Dominic Steinitz wrote:

In the crypto package, I have two functions

...

encrypt :: AESKey a => a -> Word128 -> Word128 decrypt :: AESKey a => a -> Word128 -> Word128

but the class AESKey is not exported, to prevent the user from adding more instances to it. Since AESKey is not exported, the users cannot write signatures that mention that class constraint. The question is, how to export a _closed_ class -- a class the user cannot extend. Chung-chieh Shan and I had a similar problem in our TFP2007 paper: the classes that implement type-level arithmetic must be closed, to prevent the user from declaring that 1+1=1. It seems the best method is to use class aliases: In the implementation of AES, introduce class AESKey a => AESKeyExport a instance AESKey a => AESKeyExport a -- the only instance and now export AESKeyExport (but leave AESKey hidden). If the user needs to write the signature for their function: foo :: AESKeyExport a => a -> Word128 -> Word128 foo x y = encrypt x y they should use the constraint AESKeyExport a. That constraint entails AESKey a, which they cannot mention. The users can try to extend the class ESKeyExport a, but that would do them no good: the library uses the instances of AESKey rather than AESKeyExport. The TFP2007 paper explains that in a bit more detail: http://okmij.org/ftp/Computation/resource-aware-prog/tfp.pdf (Section 2.1). It also mentions a different method, relying on partial type signatures.