[Haskell-cafe] looking for examples of non-full Functional Dependencies

Lennart Augustsson lennart at augustsson.net
Fri Apr 18 13:36:31 EDT 2008

I've never thought of one being shorthand for the other, really.
Since they are logically equivalent (in my interpretation) I don't really
care which one we regard as more primitive.

On Fri, Apr 18, 2008 at 9:26 AM, Martin Sulzmann <martin.sulzmann at gmail.com>

> Lennart Augustsson wrote:
> > To reuse a favorite word, I think that any implementation that
> > distinguishes 'a -> b, a -> c' from 'a -> b c' is broken. :)
> > It does not implement FD, but something else.  Maybe this something else
> > is useful, but if one of the forms is strictly more powerful than the other
> > then I don't see why you would ever want the less powerful one.
> >
> >  Do you have any good examples, besides the contrived one
> class D a b c | a -> b c
> instance D a b b => D [a] [b] [b]
> where we want to have the more powerful form of multi-range FDs?
> Fixing the problem who mention is easy. After all, we know how to derive
> improvement for multi-range FDs. But it seems harder to find agreement
> whether
> multi-range FDs are short-hands for single-range FDs, or
> certain single-range FDs, eg a -> b and a -> c, are shorthands for more
> powerful
> multi-range FDs a -> b c.
> I clearly prefer the latter, ie have a more powerful form of FDs.
> Martin
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