[Haskell-cafe] Base classes can be _ELIMINATED_ with interfaces

[Shelby Moore]

Shelby Moore wrote:
> ...A "type class" is a polymorphic
> (relative to data type) interface, and the polymorphism is strictly
> parameterized for the client/consumer of the interface, i.e. the data type
> is known to the function that inputs the interface AT COMPILE TIME.

>...A problem with virtual (runtime pointer) inheritance is that it hides
> the subclass from the compiler.


I emphasize that in Haskell, the consuming function knows the interface at
compile time (or it can allow the compiler to infer it, if no type class
restriction is specified).


> ...if one means by "style" the
> conflation of the interface with the data and/or use of virtual (runtime)
> base class inheritance and the style of that induces, then it is an
> architectural mistake...

One explanation of how "extends" (base class) does impact composability:

http://www.haskell.org/pipermail/haskell-cafe/2009-October/068337.html

Instead encapsulate ("inherit") data type orthogonal to "type class"
(interface) in Haskell:

http://www.haskell.org/pipermail/haskell-cafe/2009-October/068328.html
> The point of that whole rant is that extending data-bearing classes
> isn't necessarily a good idea, so before trying to find a way to do it
> with haskell, it may be better to just encapsulate another data type,
> which is trivial:
>
> data InnerThing = A | B | C
>
> data OuterThing = Outer { innerThing :: InnerThing, otherField :: Int }


Additionally note:

Shelby Moore wrote:
> In Haskel, subtyping is done with Module...

data is analogous to a public class members.  Use Module to make some
implementation private:

http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/modules.html#sect9.2

Thus use type classes (interfaces) when consuming the public Module
interfaces in functions (i.e. only consume the exported Module interfaces
in the sense they implement a type class, never directly consume the
Module name space).

I realize some examples would make the above easier to visualize.