[Haskell-cafe] Trouble understanding records and existential types

[Udo Stenzel]

John Ky wrote:
> On 1/25/07, BBrraannddoonn SS.. AAllllbbeerryy KKFF88NNHH <_a_l_l_b_e_r_y_@_e_c_e_._c_m_u_._e_d_u> wrote:
>      I'm probably missing something, but:
> 
>      (a) Why not:
> 
>      data ANode = Branch { name :: String, description :: String,
>      children :: [AnyNode] }
>                  | Leaf { name :: String, value :: String } -- this reuse
>      is legal
>      -- leaving Node available if you still need it
> 
> Would I be able to this?
> 
>    getLeaves :: ANode -> [Leaf]


data Branch = Branch { name :: String, description :: String, children :: [AnyNode] }
data Leaf   = Leaf { name :: String, value :: String }

data AnyNode = Either Branch Leaf


Now if you absolutely insist on overloading the 'name' identifier, you
can do this:


data Branch = Branch { brName :: String, description :: String, children :: [AnyNode] }
data Leaf   = Leaf { lName :: String, value :: String }

data AnyNode = Either Branch Leaf

class HasName a where name :: a -> Name
instance HasName Branch where name = brName
instance HasName Leaf where name = lName
instance HasName AnyNode where name = either brName lName


Okay, you lose record update and construction syntax for AnyNode, but I
don't think that's so much of a loss.

On a side note, all this has nothing to do with OOP.  If you wanted to
simulate objects, you would "replace case by polymorphism", but I can't
demonstrate how to do that, since none of your "objects" has any
methods.


-Udo.
-- 
"Technology is a word that describes something that doesn't work yet."
	-- Douglas Adams, JavaOne keynote, 1999
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