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