Scala: difference between a typeclass and an ADT?

Question

What are the differences between typeclasses and Abstract Data Types?

I realize this is a basic thing for Haskell programmers, but I come from a Scala background, and wo

Answer 1

The difference between a type class and an ADT is:

• Use type classes when you want to dispatch a method based off of something's type
• Use ADTs when you want to dispatch a method based off of something's value

For example, consider the print function:

print :: (Show a) => a -> IO ()

Types are static and cannot change throughout the lifetime of a program, therefore when you use a type class the method you use is chosen statically at compile time based on the inferred type at the call site. So in this example I know that I am using the Char instance for Show without even running the program:

main = print 'C'

ADTs let you change a function's behavior dynamically. For example, I could define:

print2 :: Either Char String -> IO ()
print2 (Left  c  ) = putStrLn [c]
print2 (Right str) = putStrLn str

Now, if I call print2 in some context:

print2 e

... I can't know which branch the print2 takes unless I know the runtime value of e. If the e is a Left then I take the Left branch and if e is a Right then I take the Right branch. Sometimes I can statically reason about which constructor e will be, but sometimes I cannot, such as in the following example:

main = do
    e <- readLn  -- Did I get a 'Left' or 'Right'?
    print2 e     -- Who knows until I run the program