[Haskell-cafe] Another idea for record field selection and betternamespace management

[Brian Hulley]

Apologies - I've noticed some mistakes corrected as follows:

Brian Hulley wrote:
>     class //x a b where
>          x : a -> b
>
>     class //FooCon a b where
>          FooCon : a -> b

    class //x a b | a -> b where            -- I think this fundep is 
correct
        x :: a->b                                    -- I can never get used 
to having to write :: instead of :

    class //FooCon a b | b -> a where
        FooCon :: a->b

> This is a lot more powerful than just OOP, because we could have
> different views of a data type, selecting out those components which
> are relevant to particular operations:
>
>        data Element = TerminalPunct | TerminalValue | NonTerminal |
> Push | Pop
>        data Action = Push | Pop
>        data Insertion = TerminalValue | NonTerminal | Push
>
> without having to artificially construct various injections...

No - these are not in fact views. A view would be given by multiple 
predicates eg (//Push a,//Pop a) => ... a ... as normal.

Regards, Brian.