[Haskell-beginners] Maybe and Just

[Joel Neely]

Hi, Shishir,

As a recovering complexity addict (i.e. Java programmer ;-), I'd put it
this way.

There are many methods (on many types) that can either succeed and return a
meaningful value, or be unable to fulfill the intent of the caller's
request. Example include indexOf on String,  get on Map, and read on a
Reader. Every time such a partial function scenario occurs, the author must
decide what to do if the intent can't be fulfilled. That freedom of choice
has led to a proliferation of techniques.


   - indexOf returns -1, which is a valid int, but not a valid position
   within a string.
   - get returns null, which is the only value common to all reference
   types that means 'I have nothing'. Of course, that also prevents one from
   storing null in a Map and simply retrieving it. Worse, if that result is
   passed along elsewhere, one can eventually receive a NullPointerException
   occurring in a completely different part of the code.
   - And, *horrors!*, read responds in a totally different way, by forcing
   the caller to "jump" somewhere.


Maybe provides a general, reusable solution to all such cases.

   - It allows one to "stretch" any type to include an extra "out of band"
   value.
   - The type declaration clearly (uniformly) notifies anyone else that the
   function may have to return "lack of result".
   - The type system prevents a caller from simply assuming that a result
   is present, and then accidentally propagating failure.
   - Because Maybe is an instance of Functor, one can use map to safely
   apply a function (or do nothing gracefully) without littering the calling
   code with if-statements.
   - etc.


I don't think of Maybe as enabling a *computation* that is impossible
otherwise, but I do regard it as providing a *consistency and simplicity*
that is otherwise not present.

Hope this helps,
Joel


On Thu, Mar 26, 2015 at 5:06 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Hi,
>
> After reading and re-reading the haskell tutorials I don't happen to see a
> very convincing or appealing reason for having these data types.
>
> Can anyone please explain where Maybe and Just provide the sort of
> functionality that cannot be achieved in other languages which don't have
> these kind.
>
> Thanks,
> Shishir Srivastava
>
>
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-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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