[Haskell-cafe] One-element tuple

[Mike Ledger]

It seems to me that this is Identity given a different name. A bonus of
using Identity is that it won't introduce any new packages to the majority
of installations.
On 20/08/2013 1:17 PM, "Ivan Lazar Miljenovic" <ivan.miljenovic at gmail.com>
wrote:

> On 20 August 2013 11:07, AntC <anthony_clayden at clear.net.nz> wrote:
> >> Daniel F <difrumin <at> gmail.com> writes:
> >>
> >> Can you please elaborate why this inconsistency is annoying and what's
> > the use of OneTuple?
> >> Genuine question,
> >
> > Hi Daniel, the main annoyance is the verbosity (of using a data type and
> > constructor), and that it no longer looks like a tuple.
> >
> > The inconsistency is because a one-element tuple is just as cromulent as
> a
> > n-element, or a zero-element. (And that a one-element tuple is a distinct
> > type from the element on its own/un-tupled.)
>
> Why is it as "cromulent" (especially as I'm not so sure we could
> really consider () to be merely a zero-element tuple)?
>
> I can see what you're trying to do here, but for general usage isn't a
> single element tuple isomorphic to just that element (which is what
> newtypes are for if you need that distinction)?
>
> >
> > So if I have instances (as I do) like:
> >
> >     instance C (a, b) ...
> >     instance C ()     ...
> >
> > I can't usefully put either of these next two, because they're equiv to
> > the third:
> >
> >     instance C (( a )) ...
> >     instance C ( a )   ...
> >     instance C a       ...   -- overlaps every instance
> >
> > Similarly for patterns and expressions, the so-called superfluous parens
> > are just stripped away, so equivalent to the bare term.
> >
> > The use of OneTuple is that it comes with all Prelude instances pre-
> > declared (just like all other tuple constructors). I don't see that it
> has
> > an advantage over declaring your own data type(?) I'd also be interested
> > to know who is using it, and why.
>
> As far as I'm aware, it's just a joke package, but two packages
> dealing with tuples seem to use it:
> http://packdeps.haskellers.com/reverse/OneTuple
>
> >
> > What I'm doing is building Type-Indexed Tuples [1] mentioned in HList
> [2],
> > as an approach to extensible records [3], on the model of Trex [4] -- all
> > of which acknowledge one-element records/rows/tuples. And then I'm using
> > the tuples as a platform for relational algebra [5] with natural Join
> (and
> > ideas from Tropashko's 'Relational Lattice' [6]).
> >
> > Is there anybody using OneTuple 'in anger'?
> >
> > AntC
> >
> > [1] M. Shields and E.Meijer. Type-indexed rows. In Proceedings
> >     of the 28th ACM SIGPLAN-SIGACT symposium on Principles
> >     of Programming Languages, pages 261–275. ACMPress, 2001.
> > [2] http://hackage.haskell.org/package/HList
> > [3] http://www.haskell.org/haskellwiki/Extensible_record
> > [4] http://web.cecs.pdx.edu/~mpj/pubs/polyrec.html
> > [5] http://en.wikipedia.org/wiki/Relational_algebra#Natural_join_
> > [6] http://vadimtropashko.wordpress.com/relational-lattice/
> >
> >>
> >>
> >>
> >> On Fri, Aug 16, 2013 at 5:35 AM, AntC <anthony_clayden <at>
> > clear.net.nz> wrote:
> >> There's an annoying inconsistency:
> >>     (CustId 47, CustName "Fred", Gender Male)  -- threeple
> >>     (CustId 47, CustName "Fred)                -- twople
> >> --  (CustId 47)                                -- oneple not!
> >>     ()                                         -- nople
> >
> >
> >
> >
> >
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>
>
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> http://IvanMiljenovic.wordpress.com
>
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