Concrete data consists of constructions that can be inspected, taken apart, or joined to form larger constructions. Lists are an example of concrete data. We can test whether or not a list is empty, and divide a non-empty list into its head and tail. New elements can be joined to a list. This chapter introduces several other forms of concrete data, including trees and logical propositions.

The ml datatype declaration defines a new type along with its constructors. In an expression, constructors create values of a datatype; in patterns, constructions describe how to take such values apart. A datatype can represent a class consisting of distinct subclasses — like Pascal's variant records, but without their complications and insecurities. A recursive datatype typically represents a tree. Functions on datatypes are declared by pattern-matching.

The special datatype exn is the type of exceptions, which stand for error conditions. Errors can be signalled and trapped. An exception handler tests for particular errors by pattern-matching.

Chapter outline

This chapter describes datatypes, pattern-matching, exception handling and trees. It contains the following sections:

The datatype declaration. Datatypes, constructors and pattern-matching are illustrated through examples. To represent the King and his subjects, a single type person comprises four classes of individual and associates appropriate information with each.

Exceptions. These represent a class of error values. Exceptions can be declared for each possible error.