Re: [Haskell-beginners] Equivalence of Inheritance

14 Dec 2010

      I often find myself in a similar predicament, become frustrated, and
after I've wasted a lot time ultimately decide that I'm making perfect
the enemy of good.

One possible option that hasn't been mentioned is to try and enforce
safety at the module boundary, perhaps by making MensGroup a newtype
and instead of exporting the constructor to export some other function
that ensures only men wind up in a MensGroup. Sounds cumbersome to me,
though.

-Mike

On Tue, Dec 14, 2010 at 3:53 PM, Russ Abbott  wrote:
...
Ozgur pointed out this notition p@Man{}.  What I would like is something
like this.
type Man = Person@Man{}
type MensGroup = [Man]
-- Russ
On Tue, Dec 14, 2010 at 1:45 PM, Michael Katelman  wrote:
...
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 
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 
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 
wrote:
...
Perhaps this?
https://gist.github.com/741048
-Mike
On Tue, Dec 14, 2010 at 2:27 PM, Russ Abbott 
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

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
> 
> 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
> > 
> > wrote:
> >>
> >> On Tue, Dec 14, 2010 at 1:52 PM, Russ Abbott
> >> 
> >> 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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