[Haskell-cafe] Why Maybe exists if there is Either?
Patrick Hurst
lightquake at amateurtopologist.com
Thu Jan 9 20:57:58 UTC 2014
Why have Bool? Just let true = 1, false = 0, (||) = (+), (&&) = (*).
Why have Ordering? Just use Integer and let lt = -1, eq = 0, gt = 1.
Why have three-tuples (a, b, c)? Just use ((a, b), c).
Why have Data.Map a b? Just use a -> Maybe b. You don't even need an Ord
constraint any more!
Why have Data.Set a? Just use a -> Bool (or, a -> Integer).
For that matter, why use algebraic data types? data Person = Person String
Int is isomorphic to type Person = (String, Int).
On Thu, Jan 9, 2014 at 9:50 AM, Vlatko Basic <vlatko.basic at gmail.com> 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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