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

[Vlatko Basic]

Hi Artyom,

Yes, indeed.

But instance declarations wouldn't be needed because we already have all the 
instances for Either. The point would be to have them unified.
But it is a problem that there are no data constructor synonyms in Haskell. So 
at least pattern matching wouldn't be possible the simple way. Maybe 
ViewPatterns could (partially) solve that.


vlatko

-------- Original Message  --------
Subject: Re: [Haskell-cafe] Why Maybe exists if there is Either?
From: Artyom Kazak <yom at artyom.me>
To: haskell-cafe at haskell.org
Date: 09.01.2014 16:23

> Besides, even defining
>
> type Maybe a = Either () a
>
> in standard library wouldn’t be the same as
>
> data Maybe a = Nothing | Just a
>
> since in Haskell 98 type synonyms aren’t allowed in instance declarations, which
> means that programmers would still have to remember that `Maybe` is actually
> `Either ()` under-the-hood every time when writing an instance.
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