On 20 August 2013 11:07, AntC
Daniel F
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
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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