Programming Languages: Concepts and Implementation

B.6 Lists

The following are the some important points about lists in ML.

• Unlike in Scheme, lists in ML are homogeneous, meaning all elements of the list must be of the same type. For instance, the list [1,2,3] in ML is homogeneous, while the list (1 "apple") in Scheme is heterogeneous.

• In a type-safe language like ML the values in a tuple (Section B.7) generally have different types, but the number of elements in the tuple must be fixed. Conversely, the values of a list must all have the same type, but the number of elements in the list is not fixed.

• The semantics of the lexemes nil and [] are the empty list.

• The cons operator, which accepts an element (the head) and a list (the tail), is :: (e.g., 1::2::[3]) and associates right-to-left.