How to implement Either_vt?

[Kiwamu Okabe]

Hi all,

Haskell language has Either type that represents values with two possibilities:
a value of type Either a b is either Left a or Right b.

http://hackage.haskell.org/package/base-4.7.0.2/docs/Prelude.html#t:Either

How to implement Either_vt on ATS2?

Best regards,
--
Kiwamu Okabe at METASEPI DESIGN

[Shea Levy]

Check out http://ats-lang.sourceforge.net/LIBRARY/prelude/SATS/DOCUGEN/HTML/option_vt.html for how option_vt is implemented, it is the dataviewtype equivalent of Haskell’s Maybe. either_vt would take two type parameters, and instead of Some and None it would have Left and Right constructors.

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[gmhwxi]

In ATS, there are either_p (por) and either_v (vor) for props and views, respectively.

datavtype
either_vt
(
  a1:vt@ype+
, a2:vt@ype+
, bool (*left/right:true/false*)
) =
  | inj_left(a1, a2, true) of a1
  | inj_right(a1, a2, false) of a2

vtypedef
Either_vt(a1:vt@ype, a2:vt@ype) = [b:bool] either_vt(a1, a2, b)

Surprisingly, the need for either_vt is very rare in practice. So it is not included in ATS at this moment.

[Kiwamu Okabe]

Hi Hongwei,

On Sun, Feb 22, 2015 at 11:54 PM, gmhwxi <gmh...@gmail.com> wrote:
> datavtype
> either_vt
> (
> a1:vt@ype+
> , a2:vt@ype+
> , bool (*left/right:true/false*)
> ) =
> | inj_left(a1, a2, true) of a1
> | inj_right(a1, a2, false) of a2
>
> vtypedef
> Either_vt(a1:vt@ype, a2:vt@ype) = [b:bool] either_vt(a1, a2, b)

Thank's. It's useful for me.
Because sometimes I have wanted to return an errno or a result.
However,,,

> Surprisingly, the need for either_vt is very rare in practice. So it is not
> included in ATS at this moment.

I think I understand your option_v is better than the Either_vt.
It doesn't touch the malloc heap.

Thank's,

[Kiwamu Okabe]

Hi Hongwei,

On Mon, Feb 23, 2015 at 1:56 PM, Kiwamu Okabe <kiw...@debian.or.jp> wrote:
>> Surprisingly, the need for either_vt is very rare in practice. So it is not
>> included in ATS at this moment.
>
> I think I understand your option_v is better than the Either_vt.
> It doesn't touch the malloc heap.

I miss-understand it. It's not "option_v" and "opt".