[Haskell-cafe] One-element tuple

[Dan Burton]

For the "consistency" you want, `data Oneple a = T a` is the best you can
do in Haskell.

T(CustId 47) is just one character off from what you actually want to
write: (Cust 47). And I presume you want the "extra bottom" that comes with
this, as opposed to just treating values as their own one-tuples.

I imagine you could write some fancy hack that uses the type system to
automatically promote values to Oneples of the given value when an
"expected: Oneple Foo, actual: Foo" error occurs. But this would not be
very useful in general.

An uglier option:

type Oneple a = (a, ())


-- Dan Burton


On Thu, Aug 15, 2013 at 7:05 PM, Ivan Lazar Miljenovic <
ivan.miljenovic at gmail.com> wrote:

> On 16 August 2013 11:35, 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
> >
> > (That is, it's annoying if you're trying to make typeclass instances for
> > extensible/contractable tuples. Yes, I know I could use HLists.)
> >
> > I'm not happy with either approach I've tried:
> >
> >     data Oneple a = Oneple a                   -- (or newtype)
> >     (Oneple $ CustId 47)                       -- too verbose
> >
> >     type Oneple a = [a]
> >     [CustId 47]                  -- at least looks bracket-y
> >
> > What do you do?
>
> http://hackage.haskell.org/package/OneTuple :p
>
> If you really wanted some form of parentheses you could possibly use
> quasiquoting for it...
>
> >
> > AntC
> >
> >
> >
> > _______________________________________________
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>
>
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> http://IvanMiljenovic.wordpress.com
>
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