Infix functions are functions where the function name comes in between its arguments when used. For example, operators like + and - are actually infix functions. Conversely, you can think of infix functions as operators. Here’s an example of an infix function that concatenate two vectors.
To use it, we simply treat it as any other operators.
Here’s another example of an infix function that allows you to return a default value when encountering NULL. This is very helpful when used inside of another function to avoid returning a NULL value.
To learn more, read the chapter on functions from Hadley’s book, Advanced R.