infixl vs infixr vs infix describe on which sides the parens will be grouped. For example, consider the following fixity declarations (in base)
infixl 6 -
infixr 5 :
infix 4 ==
The infixl tells us that - has left associativity, which means that 1 - 2 - 3 - 4 gets parsed as
((1 - 2) - 3) - 4
...
The number that follows the associativity information describes in what order the operators are applied. It must always be between 0 and 9 inclusive. This is commonly referred to as how tightly the operator binds. For example, consider the following fixity declarations (in base)
infixl 6 +
infixl ...
