Pairs and Tuples SML has a built-in type of pairs . You can pair together any two values. Useful for passing multiple arguments to a function, or returning multiple results. Lists SML also has a built-in data type of lists , i.e., sequences of zero or more values. You can make lists of anything, but everything in a given list must have the same type.
Lists have two interchangeable representations.
The other way to write a list is to write the elements within square brackets, separated by commas, as illustrated in the values echoed by the interactive system.