The Java type system is a lot less rich. You can take an object and call methods on it, knowing only that it implements an interface. In Haskell, you can do so much more. You don't even need an implementation, you can rely on the type of the binding instead:

extremes :: Bounded b => Bool -> b

extremes lower = if lower then minBound else maxBound

-- e.g.

extremes True :: Int -- -9223372036854775808

data Foo = A | B | C | D deriving (Bounded, Show)

extremes False :: Foo -- D

Or (with a few language extensions), you can have the typeclass take multiple arguments. In Java, you can't do anything like this:

class Convert a b where

convert :: a -> b

instance Show a => Convert a String where

convert = show

instance Convert Foo String where

convert A = "A"

convert B = "B"

convert C = "C"
convert D = "D"

Peter

On 31 January 2013 23:36, Mateusz Kowalczyk <fuuzetsu@fuuzetsu.co.uk> wrote:

Greetings,

I often wonder how one would explain type classes to someone coming from
an environment such as Java. Whenever I think about type classes, I seem
to think of them as Java interfaces. Bah, even the bottom of [1] states
> Haskell classes are roughly similar to a Java interface. Like an
interface declaration, a Haskell class declaration defines a protocol
for using an object rather than defining an object itself.

Is there more to this `roughly similar' statement? Syntax is an obvious
difference but beyond that, I can't think of anything I can do with a
Haskell type class that I wouldn't be able to do with similar amount of
effort with a Java interface, except for the fact that the interface
would look absolutely disgusting syntax wise.

Any insight appreciated.

[1] - http://www.haskell.org/tutorial/classes.html

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