[Haskell-beginners] Maybe and Just

Fri Mar 27 10:14:39 UTC 2015

Thanks Corentin. I think it makes more sense now that you've mentioned the
fact that '*Just*' in the type definition of '*Maybe*' is only a *tag*.

So would it be correct to re-define Maybe data type as follows -

data Maybe tp = Justin tp | Nothing

and then create the type value of *Maybe *as -

*Just 3*

*?*

Shishir Srivastava
 +44 (0) 750 127 5019

On Fri, Mar 27, 2015 at 10:05 AM, Corentin Dupont <corentin.dupont at gmail.com
> wrote:

>
> You're right, "Just" is used at two different levels, it's just that it
> have the same name is both cases.
> One is at type level, the other at value level.
> Since this is two separated levels, there is no risk of confusion and
> having the same name is fine.
>
> To be clear, you can write:
>
> data Maybe = Just Int | Nothing
>
> Then use it as:
>
> my_value = Just 3
>
> It the first case, "Just" is a tag in the type definition, in the second
> case it's a function constructing a value.
> They just happen to have the same name (a choice of Haskell language).
>
>
> On Fri, Mar 27, 2015 at 10:44 AM, 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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