Records (was Re: [Haskell] Improvements to GHC)

[Malcolm Wallace]

David Roundy <droundy at abridgegame.org> writes:

> > 7. Unordered records: yep (if I understand the problem correctly)
> 
> I don't think you understood correctly.  What I'd like (and this is another
> one of those David-specific issues--I've never heard anyone else complain
> about this) is to be able to create a data type that has no order.

FWIW, there are certainly other people out there who think the same
way.  In the Blobs diagram editor for instance, there are lots of
instances of exported datatypes with exported field accessors/updaters,
but the constructors are not exported.  e.g.

    module M (FooBar(), emptyFooBar, getFoo, getBar, setFoo, setBar) where
      data FooBar = FooBar { foo :: String, bar :: Bool }
      emptyFooBar = FooBar {}
      getFoo = foo
      getBar = bar
      setFoo f fb = fb {foo=f}
      setBar b fb = fb {bar=b}

This gives you three-quarters of your desire, i.e. unordered
construction is possible, positional construction is not, and
positional de-construction (pattern-matching) is also unavailable.
The only thing lacking is the ability to do unordered pattern-matching,
which is impossible here because the accessors are pure functions,
not true field names.

Just as for you, the intent of the design pattern here is total data
encapsulation - to be able to change the internals of the FooBar type
without any of the importing modules needing to change as a result.

Regards,
    Malcolm