On Mon, Mar 18, 2013 at 11:43 AM, Răzvan Rotaru
It seems that type constructors don't allow usage of typeclasses. Is it not possible to create polymorphic types in Haskell by declaration?
Not directly, no. It is possible to use existentials to do this — at the cost that you lose the ability to do anything but what you have declared about it. For example, if you were to use this to make a map of Show-able values, the *only* thing you can do with the value is invoke "show" on it (in particular: you cannot pull out the value directly, because the only thing you know about it is that "show" works on it; you otherwise have no idea what it is!). And with such a value, referential transparency means there is no difference between this and storing the *result* of invoking "show" on it (that is, a String) instead, and laziness means that it may well actually store a thunk that will invoke "show" when the value is needed instead of computing and storing the value, so that there is in fact no difference between the two except that the type is much more complicated and both the type and the code are correspondingly more difficult to understand. What problem are you trying to solve by obfuscating things in this manner? Also note that most times when someone is trying to do this, they have confused typeclasses with OOP; they are not, and trying to treat them as such *will* lead you into dead ends. -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net