Re: [Haskell-cafe] One-element tuple

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@gmail.com> wrote:

On 16 August 2013 11:35, AntC 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@gmail.com http://IvanMiljenovic.wordpress.com

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