[Haskell-beginners] Maybe and Just

[Shishir Srivastava]

ok..but what's with using the keyword 'Just' ? why cannot 'Maybe' be
defined like this

data Maybe a = a | Nothing

what's the point in having 'Just' keyword ?

Shishir


On Thu, Mar 26, 2015 at 10:26 AM, Michael Alan Dorman <
mdorman at ironicdesign.com> wrote:

> Shishir Srivastava <shishir.srivastava at gmail.com> writes:
> > 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.
>
> To be clear: Maybe is the data *type*.  Just and Nothing are its data
> *constructors*.
>
> > 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.
>
> The functionality can be achieved in other languages, certainly.  The
> question is whether the clarity and safety is also achieved.
>
> When I see (as a totally contrived example):
>
>   fopen :: Maybe FileHandle
>
> I know that that function may not be able to return a FileHandle value
> all the time.  The compiler will, in fact, nag me if I do not write the
> code that calls it in such a way that it acknowledges that possibility.
>
> When I see:
>
>   FILE * fopen ( const char * filename, const char * mode );
>
> It is not immediately clear whether that can fail.  Sure, we can make
> that inference, based on what we know about filesystems, etc., but the
> compiler is never going to complain if I ignore the possibility.
>
> In my experience, programmers in many languages end up resorting to
> convention to try and work around these sorts of ambiguities.  Large
> projects have strategies for naming functions that try to pass along
> information out of band, or languages have a pervasive culture of "lint"
> tools that try to use heuristics to make up for what the type system
> doesn't make simple.
>
> That said, I know that doing Maybe sorts of things in languages that
> don't have, say, pattern matching, or the idea of a "failure monad",
> gets to be a drag very quickly---manually unwrapping things is at best
> awkward, having to re-wrap them just to unwrap them again in a sequence
> of computations quickly leads one to believe "it's just not worth
> it"---or you resort to exception handling, which has its own challenges
> to do well.
>
> Mike.
>
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