On Jul 24, 2010, at 10:59 AM, Patrick Browne wrote:
class C1 c1 where
age :: c1 -> Integer
-- add default impl, can this be defined only once at class level?
-- Can this function be redefined in a *class* lower down the
heirarchy?
age(c1) = 1
Yes, but keep in mind that the hierarchy is only two levels tall.
This mechanism isn't meant for OO-style inheritance.
-- Is it true that instances must exists before we can run function or
make subclasses?
instance C1 Person where
instance C1 Employee where
Yes, absolutely.
-- Is it true that C2 can inherit age, provided an instance of C1
exists
class C1 c2 => C2 c2 where
name :: c2 -> String
name(c2) = "C2"
instance C2 Person where
instance C2 Employee where
There's no notion of "inheritance" here. If Person belongs to C2,
then it "must" belong to C1, because you have specifically said that a
C2 needs to be a C1 (presumably because you need a person's age to
compute their name). So Person will be using C1's "age" function, in
virtue of having a C1 instance.
Compare this to:
class C4 c4 where name' :: c4 -> String
instance C1 Person -- gives Person an age function, either default
or overridden
instance C1 thing => C4 thing -- gives every C1 thing a name, needs
Haskell extensions.
-- Is it true that C3 cannot override C1 or C2 existing defaults?
-- Is it true that this must be done at instance level?
-- class Cx c3 => C3 c3 where
-- age(c3) = 3
Yes, as I said, the hierarchy is two levels tall.
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