[Haskell-beginners] Maybe and Just

Fri Mar 27 19:01:56 UTC 2015

Shishir,

Would I be right to guess that you're coming from an OO background? If so,
I'd suggest thinking about the difference between declaring a constructor
and invoking it.

I suggest that you can read the Haskell declaration

data Maybe tp = Just tp | Nothing

as declaring a data type and specifying two constructors that can return
an instance (in the OO sense of the word) of that data type. But when the
expression

Just "FOO"

is invoking one of those constructors on a value of type String, so the
resulting value has type Maybe String.

Because Maybe is an instance of Show (in the Haskell sense of the word),
there's a show function that can be applied to that value to produce
something that looks like the line above. That's roughly equivalent to
having a toString method on a Java class that returns a string that looks
like a constructor call that could produce the actual object.

-jn-

On Fri, Mar 27, 2015 at 5:14 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> 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.
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Beginners mailing list
>>>>>>> Beginners at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>>
>
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-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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