[Haskell-beginners] Maybe and Just

[akash g]

Think of the Maybe type as

-- Valid syntax with GADTs, I think
data Maybe a where
   Nothing :: Maybe a
   Just       :: a -> Maybe a

You are confusing types with actual values :  (Just 3) is a value with
type, (Maybe Int).

Think of it this way: A value is an object with some type and you have
Maybe value constructors via Nothing and Just.





On Fri, Mar 27, 2015 at 3:14 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Sorry, but I still have some grudges so to say with the way 'Maybe' is
> defined.
>
> By definition '*Maybe*' takes a type parameter which I will denote here
> as '*tp*' for the sake of clarity like below -
>
> data Maybe tp = Just tp | Nothing
>
> Therefore '*tp*' is not representing a value perse such as '3' or 'XYZ'
> etc but a data type such as '*Int*', '*Char*' etc.
>
> But when we have to create a data type of '*Maybe*' we write '*Just 3*'
> and not '*Just Int*' or '*Just Char*' which is how it's defined in the
> definition (i.e. '*Just tp*' ) .
>
> It is this discrepancy in definition and the actual value creation which
> is slightly troubling me.
>
> Thanks,
> Shishir
>
> Shishir Srivastava
>  +44 (0) 750 127 5019
>
>
> On Thu, Mar 26, 2015 at 5:31 PM, Corentin Dupont <
> corentin.dupont at gmail.com> wrote:
>
>> Not really, when you define the type "Maybe a":
>>
>> data Maybe a = Just a | Nothing
>>
>> Haskell is creating automatically two functions for you:
>>
>> Just :: a -> Maybe a
>> Nothing :: Maybe a
>>
>> In the first case, you can think of "Just" and "Nothing" as a sort of tag
>> identifying which element of the sum you have.
>> It the second case it's a function, with the same name.
>> A more informed person than me could say if they are indeed separated of
>> if they are the same thing in GHC...
>>
>>
>> On Thu, Mar 26, 2015 at 5:34 PM, Shishir Srivastava <
>> shishir.srivastava at gmail.com> wrote:
>>
>>> isn't that then cyclic dependency between 'Maybe' and 'Just' ...where
>>> the first one is defined in terms of second and vice-versa...?
>>>
>>> Shishir
>>>
>>>
>>> On Thu, Mar 26, 2015 at 3:48 PM, Corentin Dupont <
>>> corentin.dupont at gmail.com> wrote:
>>>
>>>> Hi Shishir,
>>>> I think that's a legitimate question.
>>>>
>>>> By writing
>>>>
>>>> data Maybe a = a | Nothing
>>>>
>>>> you are saying that the type of the left hand side of the = is the same
>>>> that right hand side (you are defining a type basically).
>>>> Also you can only sum things of the same type.
>>>> So you are saying:
>>>> type "Maybe a" = type "a"
>>>> Which is wrong.
>>>> That's why "a" should be wrapped into something:
>>>> type of "Just a" is indeed "Maybe a".
>>>>
>>>> "Just" is a type constructor:
>>>> Just :: a -> Maybe a
>>>> It allows you to build the Maybe.
>>>>
>>>> Said that, "Just" is a vocabulary choice.
>>>> Personally I prefer the name choices of OCaml, Rust, Scala etc.:
>>>> Option a = None | Some a
>>>>
>>>>
>>>>
>>>> On Thu, Mar 26, 2015 at 4:26 PM, Shishir Srivastava <
>>>> shishir.srivastava at gmail.com> wrote:
>>>>
>>>>> 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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>>>>
>>>
>>
>
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