Concepts of Programming Languages

288 Chapter 6 Data Types

Accessing the value of a union is done with a pattern-matching structure.

Pattern matching in F# is specified with the match reserved word. The general

form of the construct is as follows:

match pattern with

| expression_list 1 - > expression 1

|...

| expression_listn-> expressionn

The pattern can be any data type. The expression list can include wild card

characters ( _ ) or be solely a wild card character. For example, consider the

following match construct:

let a = 7;;

let b = "grape";;

let x = match (a, b) with

| 4, "apple" -> apple

| _, "grape" -> grape

| _ -> fruit;;

To display the type of the intReal union, the following function could

be used:

let printType value =

match value with

| IntValue value -> printfn "It is an integer"

| RealValue value -> printfn "It is a float";;

The following lines show calls to this function and the output:

printType ir1;;

It is an integer

printType ir2;;

It is a float

6.10.5 Evaluation

Unions are potentially unsafe constructs in some languages. They are

one of the reasons why C and C++ are not strongly typed: These languages

do not allow type checking of references to their unions. On the other

hand, unions can be safely used, as in their design in Ada, ML, Haskell,

and F#.

Neither Java nor C# includes unions, which may be reflective of the growing

concern for safety in some programming languages.