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

Vlatko Basic vlatko.basic at gmail.com
Thu Jan 9 15:26:58 UTC 2014

Hi Johannes,

I thought there was some "more important" reason than convenience, so I asked. :-O)

I put String because I'm currently thinking about error handling, and Left 
String is the usual way of reporting failure, and I see Maybe as a type for 
reporting errors, failures and similar.

Somehow it looks to me that famous "8 ways to report errors in Haskell" could be 
shortened by one if Maybe is replaced with Either (with appropriate synonyms, of 


-------- Original Message  --------
Subject: Re: [Haskell-cafe] Why Maybe exists if there is Either?
From: Johannes Erber <Hannes_E at gmx.de>
To: vlatko.basic at gmail.com, haskell-cafe at haskell.org <haskell-cafe at haskell.org>
Date: 09.01.2014 16:07

> Hi Vlatko,
> to keep programs clear, short and simple. What makes you choose String as type
> to replace Nothing?
> Best,
> Johannes
> On 09/01/2014 14:50, Vlatko Basic wrote:
>> Hello Cafe,
>> With my current knowledge of Haskell, I do not see why is there Maybe if we
>> have Either.
>> For example, Functor and Monad instances (and many others) of Maybe and Either
>> are the same (except for fail).
>> In other words, this should hold:
>>     Maybe a  = Either String a     -- String or something else
>>     Nothing   = Left ""
>>     Just a       = Right a
>> 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?
>> Best regards,
>> vlatko
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