[Haskell-beginners] Type classes are not like interfaces,
Brandon S. Allbery KF8NH
allbery at ece.cmu.edu
Sat Jan 24 00:14:06 EST 2009
On 2009 Jan 23, at 8:37, Francesco Bochicchio wrote:
> I think I get the polymorphism. What I don't get is why a
> specialized type cannot
> replace a more generic type, since the specialized type implements
> the interface
> defined in the generic type.
Try it this way. The declaration
> a :: Num n => n
> a = 3 :: Integer
is not the same as
> AbstractInterface a = new ConcreteClass();
because Integer doesn't implement *all* of Num, in particular nothing
needed for the Float, Double, or Complex instances. In a sense,
instances in Haskell are inverses of interfaces in Java; in Java you
accept more specific but in Haskell you accept *less* specific. This
is because an instance actually asserts a "for all possible matching
types" requirement, whereas Java asserts an "any matching type"
> a :: forall n. Num n => n
You don't have to write (and in Haskell 98, can't write, just as in
Java you can't explicitly write "forany" in an interface definition)
the "forall"; in Haskell98 (and Java respectively) it's there by
definition, in Haskell extensions it's implied for top level types for
In many cases you can treat the two the same because the member types
happen to have an exact relationship such that "forall" and "forany"
are both satisfied, but Num is too wide (and far too complex; the
ideal structure of the Haskell numeric classes is constantly
debated). So the equivalence of typeclasses and interfaces is a "most
of the time" thing, not a definition or requirement.
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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