Operator Fixity
You can use the infix, infixl or infixr keywords to define operators associativity and precedence. Example taken from the reference:
main = print (1 +++ 2 *** 3)
infixr 6 +++
infixr 7 ***,///
(+++) :: Int -> Int -> Int
a +++ b = a + 2*b
(***) :: Int -> Int -> Int
a *** b = a - 4*b
(///) :: Int -> Int -> Int
a /// b = 2*a - 3*b
Output: -19
The number (0 to 9) after the infix allows you to define the precedence of the operator, being 9 the strongest. Infix means no associativity, whereas infixl associates left and infixr associates right.
This allows you to define complex operators to do high level operations written as simple expressions.
Note that you can also use binary functions as operators if you place them between backticks:
main = print (a `foo` b)
foo :: Int -> Int -> Int
foo a b = a + b
And as such, you can also define precedence for them:
infixr 4 `foo`
