[Haskell-cafe] class Ref...

Sven Panne Sven.Panne at aedion.de
Sun Jun 12 12:58:42 EDT 2005

ajb at spamcop.net wrote:
> Quoting Gracjan Polak <gracjan at acchsh.com>:
> [...]
>>Is there any reason why isn't it included?
> Nobody could agree on the details.  For example, MVars are perfectly
> respectable Refs on the IO monad.  So would it make sense to add an
> instance for that?  If so, the functional dependency should go, which
> introduces its own problems.

A few more design problems:

  * Due to the functional dependency, that class is not Haskell98, which
    is a *very* good reason IMHO not to standardize it, at least in that
    way. Remember: There are not only GHC and Hugs out there...

  * The 3 operations should not be packed together in a single class,
    because there might be e.g. references which you can't create (e.g.
    OpenGL's state variables), references which are read-only and even
    references which are write-only.

  * What about strictness of e.g. the setter? There is no "right" version,
    this depends on the intended usage.

  * Are the references located in the monad (like in the suggested class)
    or are they within objects, which have to be given as additional
    arguments (e.g. like wxHaskell's widgets/Attr/Prop).

  * Atomic operations might be needed, too.


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