[Haskell-cafe] default function definitions

[Daniel Fischer]

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.