[Haskell-cafe] Why Maybe exists if there is Either?

[Nickolay Kudasov]

Actually, Left absurd is perfect Nothing (mind laziness!).

On the other hand, with Either () a you have 2 different Nothings:
Left ()and Left
_|_.


2014/1/10 Dan Frumin <difrumin at gmail.com>

>
>
> On 10 Jan 2014, at 04:17, John Lato <jwlato at gmail.com> wrote:
>
> On Thu, Jan 9, 2014 at 8:51 AM, Artyom Kazak <yom at artyom.me> wrote:
>
>>
>> On 01/09/2014 08:36 PM, Vlatko Basic wrote:
>>
>>> But instance declarations wouldn't be needed because we already have all
>>> the instances for Either. The point would be to have them unified.
>>>
>> Consider a generic Show instance for Either – there’s no way¹ to make it
>> behave differently for Either () (or Maybe if it was a type synonym). I
>> think there are other cases in which we want Maybe to behave differently
>> from Either, but I can’t think of any on the spot.
>>
>> ¹ this point also applies to String, which is a type synonym for [Char] –
>> and it was solved somewhat inelegantly by adding an additional method to
>> Show typeclass *specifically* for showing lists of things.
>
>
> I think this is a really important point.  If I may go further, it's
> arguable that 'type String = [Char]' was a poor decision, and one of the
> arguments is that it's not possible to make different instances for String
> and [a] (hence the showsList issue).
>
> Also, due to laziness, Either () a is bigger than Maybe a.  It should be
> 'Either Void a'.   But Void has only recently been added to the
> standard-ish library, which means if we'd used Either () from the start now
> we'd be stuck with the wrong type.  I'd rather have a separate Maybe that
> does exactly what it's meant to.
>
>
> Why is 'Either Void a' isomorphic to 'Maybe a'. What would 'Nothing' be in
> this case?
>
> Either () a is the correct version. Roughly speaking 'Maybe a' contains
> |a|+1 values while Either Void a contains only |a| values, since you can
> not construct any Left's
>
>
> John L.
>
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