Re: [Haskell-cafe] type classes and logic

On Friday 27 August 2010 14:54:53, Patrick Browne wrote:

class BEING human  => HUMAN human where

At this point there is no additional functionality is defined for the subclass HUMAN Sub-classing is logical implication BEING(human)  => HUMAN(human) All types t that make BEING(t) = true also make HUMAN(t)=true

No, it's the other way round. Every HUMAN is also a BEING, hence HUMAN(t) => BEING(t) Admittedly, the notation for subclasses in Haskell is backwards. The corresponding situation for Java interfaces (which are roughly analogous to type classes) would be interface BEING{ ... } interface HUMAN extends BEING{ ... }