What does the => symbol mean in Haskell?

Question

I\'m new to Haskell and, in general, to functional programming, and I\'m a bit uncomfortable with its syntax.

In the following code what does the =>

Answer 1

One way to think about it is that Ord a and Num a are additional inputs to the function. They are a special kind of input though: dictionaries. When you use this function with a particular type a, there must also be dictionaries available for the Ord and Num operations on the type a as well.

Any function that makes use of a function with dictionary inputs must also have the same dictionary inputs.

foo :: (Num a, Ord a) => a -> t
foo x = loop x someFunc someT

However, you do not have to explicitly pass these dictionaries around. Haskell will take care of that for you, assuming there is a dictionary available. You can create a dictionary with a typeclass instance.

instance Num MyType with
  x + y = ...
  x - y = ...
  ...

This creates a dictionary for the Num operations on MyType, therefore MyType can be used anywhere that Num a is a required input (assuming it satisfies the other requirements, of course).