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

[Patrick Hurst]

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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