On Saturday 24 July 2010 19:59:26, Patrick Browne wrote:
module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show
class C1 c1 where age :: c1 -> Integer -- add default impl, can this be defined only once at class level? -- Can this function be redefined in a *class* lower down the heirarchy? age(c1) = 1
That would normally be written age _ = 1 Yes, a default implementation of a class method can only be given in the class definition, it can't be redefined, but it can be overridden in an instance declaration. For example instance C1 Person where age _ = 3 instance C1 Employee where age (Employee _ n) = n
-- Is it true that instances must exists before we can run function or make subclasses? instance C1 Person where instance C1 Employee where
You can *call* class methods only for types which are instances of that class. You can define "subclasses" [not to confuse with OO-subclasses] of C1 even if there are no instances of C1 in scope.
-- Is it true that C2 can inherit age, provided an instance of C1 exists class C1 c2 => C2 c2 where name :: c2 -> String name(c2) = "C2"
A compiler/interpreter can accept an instance declaration of C2 for a type only if an instance of C1 for that type in scope [defined in the same module or in a module directly or indirectly imported; class instances are always re-exported from a module]. Since an instance of C2 must also be an instance of C1, all methods of C1 can be applied to that type.
instance C2 Person where instance C2 Employee where
-- Is it true that C3 cannot override C1 or C2 existing defaults? -- Is it true that this must be done at instance level? -- class Cx c3 => C3 c3 where -- age(c3) = 3 --
True. Default methods can only be overridden in instance declarations for the class.