A function returns different values depending on its input. In Gren, a function will always return the same result if provided the same input.
sumOf first second = first + second sumOfFiveAndTwo = sumOf 5 2
sumOf is the name of the function, while
second are the inputs to the function. The return value of
sumOf, is the last computed expression. In this case, the only expression is
first + second, the result of which becomes the returned value.
Functions can take an arbitrary number of arguments, but must return exactly one value.