[Haskell-beginners] Type Class Woes ..

[Chaddaï Fouché]

On Sun, Aug 30, 2009 at 2:32 PM, Patrick
LeBoutillier<patrick.leboutillier at gmail.com> wrote:
>> volume :: Fruit FruitType -> Double
>> volume F{radius=r,len=l,fType=Orange} = (4.0/3.0) * pi * r * r * r
>>
>> volume F{radius=r,len=l,fType=Apple}  = (4.0/3.0) * pi * r * r * r
>> volume F{radius=r,len=l,fType=Banana} = pi * (r * r) * l
>> volume F{radius=r,len=l,fType=Watermelon} = (4.0/3.0) * pi * (2.0 * r)
>>                                                         * l * (0.5 * l)
>
> Can anyone explain the above pattern matching syntax? I've never seen it
> before...

It's part of what record syntax allows : record pattern.

record_pattern = data_constructor '{' (field_pattern ',')* '}'
field_pattern = field_name '=' pattern

You don't have to use all the fields of the datatype in a pattern if
you don't need them all. A special case is when you put zero
field_pattern in the {}, in this case you can even use this syntax for
regular datatype (no record syntax), to write thing like :

> isJust Just {} = True
> isJust _ = False

(especially interesting for constructors with plenty of parameters, of course)

-----------

To come back to the initial subject, if you use datatypes and
typeclass rather than dataconstructors and pattern matching to allow
extensibility of data, you can still have a list of different fruits
type, using existential types, though that is not without minus :

> data Fruit a = F { radius, length :: Double }
> data Orange; data Banana;
>
> class Volume a where
>   vol :: a -> Double
>
> instance Volume (Fruit Orange) where
>   vol (F r _) = (4/3) * pi * r * r * r
> instance Volume (Fruit Banana) where
>   vol (F r l) = pi * (r * r) * l
>
> data Volumic = Volume a => V a
>
> fruit_list :: [Volumic]
> fruit_list = [V (F 3 undefined :: Fruit Orange), V (F 1 6 :: Fruit Banana) ]

In this particular case it is really uninteresting since you could as
well stock a list of volumes (the only thing you can do with a Volumic
is get the volume of its content) but with more versatile typeclass,
it may be different.

-- 
Jedaï