Can you overload + in haskell?

Question

While I\'ve seen all kinds of weird things in Haskell sample code - I\'ve never seen an operator plus being overloaded. Is there something special about it?

Let\'s

Answer 1

Overloading in Haskell is only available using type classes. In this case, (+) belongs to the Num type class, so you would have to provide a Num instance for your type.

However, Num also contains other functions, and a well-behaved instance should implement all of them in a consistent way, which in general will not make sense unless your type represents some kind of number.

So unless that is the case, I would recommend defining a new operator instead. For example,

data Pair a b = Pair a b
    deriving Show

infixl 6 |+| -- optional; set same precedence and associativity as +
Pair a b |+| Pair c d = Pair (a+c) (b+d)

You can then use it like any other operator:

> Pair 2 4 |+| Pair 1 2
Pair 3 6