At 2001-01-17 16:07, Johan Nordlander wrote:

Ashley Yakeley wrote:

...

OK, I've figured it out. In this O'Haskell statement,

...

struct Derived < Base = value :: Int

...Derived is not, in fact, a subtype of Base. Derived and Base are disjoint types, but an implicit map of type "Derived -> Base" has been defined.

-- Ashley Yakeley, Seattle WA

Well, they are actually subtypes, as far as no implicit mapping needs to be defined. But since Derived and Base also are two distinct type constructors, the overloading system treats them as completely unrelated types (which is fine, in general).

All O'Haskell treats them as completely unrelated types. In fact, this O'Haskell...

struct Base = b1 :: Int b2 :: Char

struct Derived < Base = d1 :: Int

...is a kind of syntactic sugar for this Haskell...

data Base = Base (Int,Char) dotb1 (Base (x,_)) = x dotb2 (Base (_,x)) = x

data Derived = Derived (Int,Char,Int) dotd1 (Derived (_,_,x)) = x

implicitMap (Derived (b1,b2,d1)) = Base (b1,b2)

This seems to be stretching the concept of 'subtype'. Sorry if I sound so bitter and disappointed. I was hoping for a Haskell extended with real dynamic subtyping... -- Ashley Yakeley, Seattle WA