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.miljeno...@gmail.com> wrote:
> On 20 August 2013 11:07, AntC <anthony_clay...@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 > > > > > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -- > Ivan Lazar Miljenovic > ivan.miljeno...@gmail.com > http://IvanMiljenovic.wordpress.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
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