Sebastian Fischer wrote:
Thomas DuBuisson wrote:
data Key = Key { encrypt :: B.ByteString -> B.ByteString, decrypt :: B.ByteString -> B.ByteString, keyLength :: BitLength, serialize :: B.ByteString}
rsa :: RandomGen g => BitLength -> g -> ((Key,Key), g)
One reason against this is simply that all the other constructs (block/stream cipher, hashes) are classes, it would be odd for there to be a single exception.
Chances are that you can express them like this as well. :)
A better reason is the data structure has no way to implement generateKeyPair.
That's a non-problem: each algorithm (RSA, DSA, ...) implements a function with the same type as generateKeyPair . Compare rsa :: RangomGen g => BitLength -> g -> ((Key,Key), g) vs ((k1 :: RSA, k2), g') = generateKeyPair g You always have to write down the name of the algorithm ("RSA") when using generateKeyPair , so you may as well drop it entirely.
Also, the type-class approach is extensible in that new operations (for example for signing) can be added via subclasses. Later extending the key type above requires nesting.
That's an argument in favor of type classes, indeed. However, if the extension applies to *all* key types, i.e. if you could merge the subclasses into the superclass, then you can add the new fields to the Key type. If the record fields are read-only, this will not break backwards compatibility.
In general, I like this approach, but what are
encrypt privateKey
or
decrypt publicKey
supposed to do? A type-class solution also does not *prevent* programmers to perform such non-sensical calls, but the data-type solution *forces* programmers to provide non-sensical encrypt and decrypt functions when creating the public and private keys.
Would it be desirable to prohibit such calls using the type system?
Unless the implementor of a new encryption algorithm uses different types to distinguish between public and private key, the type class approach forces him to provide non-sensical encrypt and decrypt functions, too. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com