[Haskell-beginners] Equivalence of Inheritance

[Michael Katelman]

Without going too exotic, I see two choices. Keeping the single type
Person makes what you are asking for, as far as I know, impossible
(dependent types). For human readability you could consider

type MensGroup = [Person]
type WomensGroup = [Person]

If you split Person into two types, Man and Woman, there are
repercussions for the aggregate group type

type Group = [Either Man Woman]

-Mike


On Tue, Dec 14, 2010 at 3:30 PM, Russ Abbott <russ.abbott at gmail.com> wrote:
> Now that this is straightened out, I went back to what I was doing in the
> first place and realized that I haven't solved my problem.
> Given
>
> data Person =
>       Man {name :: String, age :: Int, prostateCondition :: Condition}
>   | Woman {name :: String, age :: Int, ovaryCondition    :: Condition}
>
> I'd like to define something like this.
>
> type MensGroup = [Man]
>
> Is there a way to do something like that?
> -- Russ
>
>
> On Tue, Dec 14, 2010 at 1:06 PM, Russ Abbott <russ.abbott at gmail.com> wrote:
>>
>> That's good. (It's more or less the way I was doing it.)  What I wanted to
>> avoid was this.
>>
>> getGenderSpecificCondition (  Man _ _ cond) = cond
>> getGenderSpecificCondition (Woman _ _ cond) = cond
>>
>> I know it seems like a small thing, but I would like to be able to write
>> it like this.
>>
>> getGenderSpecificCondition p
>>    | p == (Man _ _ cond) = cond
>>    | p == (Woman _ _ cond) = cond
>>
>> But that's not legal syntax.  A pattern can't appear in that context. But
>> this does the job.
>>
>> getGenderSpecificCondition :: Person -> Condition
>> getGenderSpecificCondition p
>>    | isMan p = prostateCondition p
>>    | isWoman p = ovaryCondition p
>>
>> isMan (     Man _ _ cond) = True
>> isMan _ = False
>> isWoman (Woman _ _ cond) = True
>> isWoman _ = False
>>
>> That works! prostateCondition and ovaryCondition are both defined on
>> Person. (I'm surprised to see that.)
>>
>> *Person> Group [Man "Harry" 32 OK, Woman "Sally" 29 Good]
>> Harry(32, OK)
>> Sally(29, Good)
>>
>> Also
>>
>> *Person> prostateCondition (Woman "Sally" 29 Good)
>> *** Exception: No match in record selector prostateCondition
>> *Person> prostateCondition (Man "Harry" 29 Good)
>> Good
>>
>> -- Russ
>>
>>
>> On Tue, Dec 14, 2010 at 12:31 PM, Michael Katelman <katelman at uiuc.edu>
>> wrote:
>>>
>>> Perhaps this?
>>>
>>> https://gist.github.com/741048
>>>
>>> -Mike
>>>
>>> On Tue, Dec 14, 2010 at 2:27 PM, Russ Abbott <russ.abbott at gmail.com>
>>> wrote:
>>> > What I'm after is a version of my example that compiles.  Can you make
>>> > one?
>>> >
>>> > -- Russ
>>> >
>>> >
>>> > On Tue, Dec 14, 2010 at 12:18 PM, Antoine Latter <aslatter at gmail.com>
>>> > wrote:
>>> >>
>>> >> Sorry, I really don't know enough about what you're after to attempt
>>> >> that.
>>> >>
>>> >> But you'll need to change you're signatures of the form:
>>> >>
>>> >> > function :: Person -> Foo
>>> >>
>>> >> to something of the form:
>>> >>
>>> >> > function :: Person p => p -> Foo
>>> >>
>>> >> Because again, a type class can not be used as a type.
>>> >>
>>> >> Antoine
>>> >>
>>> >> On Tue, Dec 14, 2010 at 2:12 PM, Russ Abbott <russ.abbott at gmail.com>
>>> >> wrote:
>>> >> > What got fouled up is all the adjustments I had to make to the other
>>> >> > declarations.
>>> >> > Can you complete the example so that it compiles using
>>> >> >
>>> >> > class Person p where ...
>>> >> >
>>> >> > I'd very much like to see an example that actually compiles.
>>> >> >
>>> >> > Thanks.
>>> >> > -- Russ
>>> >> >
>>> >> > On Tue, Dec 14, 2010 at 11:58 AM, Antoine Latter
>>> >> > <aslatter at gmail.com>
>>> >> > wrote:
>>> >> >>
>>> >> >> On Tue, Dec 14, 2010 at 1:52 PM, Russ Abbott
>>> >> >> <russ.abbott at gmail.com>
>>> >> >> wrote:
>>> >> >> > If gender is a field in a Person type, then a Person must have
>>> >> >> > both
>>> >> >> > an
>>> >> >> > ovaryCondition and a prostateCondition.  That seems awkward.
>>> >> >> > Regarding
>>> >> >> >      class Person p where
>>> >> >> > I started down that path but got completely fouled up.
>>> >> >>
>>> >> >> How did this get fouled up? Every class declaration must take
>>> >> >> arguments - here, 'p' is the argument for the class.
>>> >> >>
>>> >> >> Thanks,
>>> >> >> Antoine
>>> >> >
>>> >> >
>>> >
>>> >
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>>> >
>>> >
>>
>
>