The arity of a function is the number of arguments the function takes. Functions of arity 0, 1, 2 and 3 are often called nullary, unary, binary and ternary functions, respectively. For example, the following function sqrsum1 is a binary function such that its two arguments are of the type int:
// typedef int2 = (int, int) // fn sqrsum2 (xy: int2): int = let val x = xy.0 and y = xy.1 in x * x + y * y end // end of [sqrsum2]
Many functional languages (e.g., Haskell and ML) only allow unary functions. A function of multiple arguments is encoded in these languages as a unary function taking a tuple as its only argument or it is curried into a function that takes these arguments sequentially. ATS, however, provides direct support for functions of multiple arguments. There is even some limited support in ATS for variadic functions, that is, functions of indefinite number of arguments (e.g., the famous printf function in C). This is a topic I will cover elsewhere.