data-default WAS: [containers] Proposal: Change to the Data.Map Monoid

[John Lato]

I don't find Default for things like ParserOptions that compelling at all.
IMHO Henning's approach is the best overall.

In practice the problem I have with deriving the instance generically is
that it just doesn't work. I commonly have several Bool values, and
possibly some Ints or Doubles, and they all need different defaults.  The
only way to make that work would be newtypeing over Bool and writing the
default instances by hand. No thanks, just writing a straightforward value
is much nicer.

John L.

On May 22, 2014 6:00 AM, "Roman Cheplyaka" <roma at ro-che.info> wrote:
>
> I don't agree to Empty either. As I mentioned, the most compelling use
cases for
> Default I see are things like ParserOptions and ConnectionInfo.
>
> Empty could be a separate typeclass with a different set of instances. It
should
> probably come with laws (its monofoldable length/size should be 0).
>
> Roman
>
> * João Cristóvão <jmacristovao at gmail.com> [2014-05-22 12:36:24+0100]
> > Hi Roman, Daniel,
> >
> > I do see your point, and I don't find it a problem. In those particular
> > cases perhaps it only makes sense to define a Default value for the same
> > cases as mempty applies, namely for Sum and Product.
> >
> > So, I'm fine with not including default values for Int, Bool, Float...
> >
> > However, I'm not sure I agree with calling it Empty.
> > You might want to use Default with your own Algebraic data type where
the
> > concept of Default makes sense, but the concept of Empty does not.
> > The Empty name may be a little too misleading (as mempty is already for
> > Monoid, see the Product newtype).
> >
> > Cheers,
> > João
> >
> >
> > 2014-05-22 9:10 GMT+01:00 Daniel Trstenjak <daniel.trstenjak at gmail.com>:
> >
> > >
> > > Hi João,
> > >
> > > > ... with an obvious default value ...
> > >
> > > I think that's the problem some people have, that the default values
> > > for several types aren't obvious at all and therefore you have to know
> > > the default values of several types to easily read code using 'def'.
> > >
> > > The default values for container like types - like String or List -
are
> > > mostly quite obvious, they're just the empty ones. But the default
> > > values for types like Int, Bool of Float aren't that obvious and they
> > > certainly don't make sense in any use case.
> > >
> > > That's also the reason why an Empty type class was advocated instead
> > > of the Default one, because than it's really obvious what the result
> > > for container like types is.
> > >
> > >
> > > Greetings,
> > > Daniel
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> > >
>
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