[Haskell-cafe] Attempt to emulate subclasses in Haskell, am I reinventing the wheel?

[Alexander Solla]

Don't do this.  It isn't a "bad" idea, it's just that you're not using the
language to its full potential, and will end up with a lot of annoying (and
not quite trivial) boilerplate.

Read 'Data types a la carte'.[1]  There really ought to be a "standard"
(even if unofficial) library to do open data types, but rolling your own is
really easy.

[1]: http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf

On Mon, Apr 20, 2015 at 11:01 AM, Clinton Mead <clintonmead at gmail.com>
wrote:

> I've linked to an ugly attempt to emulate subclasses in Haskell, and I'm
> wondering if anyone has done what I have perhaps in a cleaner way.
>
> Firstly, the code is here: http://ideone.com/znHfSG
>
> To explain what I've done, I first thought that a "method" basically takes
> some "input" (which I've called "i"), an object (which I've called "c" for
> class) and returns some output "o" and a potentially modified object "c".
> I've captured this behaviour in the badly named class "C".
>
> I've then made a class "User", with methods "getFirstName" and
> "putFirstName" and defined them appropriately.
>
> Furthermore, I've then made a data type "Age", and then "ExtendedUser"
> which combines "User" with "Age".
>
> At this point, I can still call "getFirstName" and "putFirstName" on
> "ExtendedUser", as would be hoped.
>
> I also defined "getAge", which naturally works on ExtendedUser.
>
> Furthermore, I can override "getFirstName" on "ExtendedUser", which I have
> done to instead return a capitalised version.
>
> Is what I've done of any practical use? And has someone done it better
> than me?
>
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