7 ms·

Care to elaborate?

• dllthomas 13y ago
  As an easy to follow (though questionably idiomatic) example, consider the typeclass Default: class Default a where def :: a and an unwise pair of instances: instance Default Int where def = 7 instance Default String where def = "foo" This means I can say: 3 + def + length ("c" ++ def) and get back 14. Note that which "def" depends on the type expected where it is used. --- It's not limited to values, either. Consider the "return" function in the Monad typeclass: return :: Monad m => a -> m a which is a function that takes something and returns that something wrapped in a monad. Which monad? Whatever is expected at the call site. [1, 2, 3] ++ return 4 gives us [1, 2, 3, 4] because list is a monad and return for lists gives a single element list, whereas putStrLn "foo" >> return 4 gives us an IO action that, when executed, prints "foo" and yeilds a 4. --- A super complex example is variadic functions like printf, with the type printf :: PrintfType r => String -> r PrintfType can be a String or IO (), giving you something like C's sprintf or printf based on the call site (which is itself cool), but it can also be a function that takes an instance of PrintfArg and gives back some new PrintfType (in an interesting intersection with currying).