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

[Ben Foppa]

I'm generally of the opinion that if things can be unified under the hood,
it makes for fewer edge cases and easier testing.

It also seems to me that I describe Maybe and Either () in the same way, if
I were to explain verbally: "it's either nothing, or it's something".

Aside from the point made about typeclass instances, I don't see an issue
with:

type Maybe = Either ()

nothing :: Maybe a
nothing = Left ()

just :: a -> Maybe a
just = Right
On Jan 9, 2014 12:02 PM, "Niklas Larsson" <metaniklas at gmail.com> wrote:

>  Maybe doesn't have to be used for error reporting. Optional values is a
> very big use case for it.
>  ------------------------------
> Från: Vlatko Basic <vlatko.basic at gmail.com>
> Skickat: 2014-01-09 16:37
> Till: haskell-cafe at haskell.org
> Ämne: Re: [Haskell-cafe] Why Maybe exists if there is Either?
>
> Hi Daniel,
>
> > If you just want to signalize a fail case without any additional
> > information, then a Maybe fits better than an Either, because
> > why should you need this bogus empty string?
>
> Now it looks to me that it might be better and more consistent to write an
> empty
> bogus string (as with: nothing = Left "") than to have two distinct ways
> of
> error reporting, and both are used widely and often should be intermixed.
> And at the end, call site could decide does it want to use the string or
> not.
>
>
> vlatko
>
>
> -------- Original Message  --------
> Subject: Re: [Haskell-cafe] Why Maybe exists if there is Either?
> From: Daniel Trstenjak <daniel.trstenjak at gmail.com>
> To: haskell-cafe at haskell.org
> Date: 09.01.2014 16:13
>
> >
> > Hi Vlatko,
> >
> > On Thu, Jan 09, 2014 at 03:50:16PM +0100, Vlatko Basic wrote:
> >> I'm curious to find out what was the reasoning to make Maybe?
> >> What is the added value with introducing it?
> >> In which situations the above substitution does not hold?
> >
> > If you just want to signalize a fail case without any additional
> > information, then a Maybe fits better than an Either, because
> > why should you need this bogus empty string?
> >
> > How should you know that the string doesn't contain something relevant?
> >
> >
> > Greetings,
> > Daniel
> > _______________________________________________
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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
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