So I wanted to understand the functions foldl and foldr in Hoogle
they say that these functions apply to binary operators. But I don't get what these binary operators are. I know that (+) and (-) are binary operators but what defines them ?
Usually mathematicians define a binary operator is a function that takes two arguments. In some papers it is a requirement that the arguments and the result are defined over the same set.
The fact that the function ? takes two arguments results in the fact that mathematicians frequently use infix notation (like x ? y). For instance (+), (*), etc. are clearly binary operators.