Editing Vienna Development Method (section)

===Type constructors: union, product and composite types===
The basic types alone are of limited value. New, more structured data types are built using type constructors.
{| border="1" class="wikitable"
|+ style="background:#ffdead;" | Basic Type Constructors
|-
| <code><nowiki>T1 | T2 | ... | Tn</nowiki></code> ||  Union of types <code>T1,...,Tn</code>
|-
| <code>T1*T2*...*Tn</code> ||  Cartesian product of types <code>T1,...,Tn</code>
|-
| <code>T :: f1:T1 ... fn:Tn</code>  || Composite (Record) type
|}

The most basic type constructor forms the union of two predefined types. The type <code>(A|B)</code> contains all elements of the type A and all of the type <code>B</code>. In the traffic signal controller example, the type modelling the colour of a traffic signal could be defined as follows:
<syntaxhighlight lang="bnf">
SignalColour =  <Red> | <Amber> | <FlashingAmber> | <Green>
</syntaxhighlight>

[[Enumerated type]]s in VDM-SL are defined as shown above as unions on quote types.

Cartesian product types may also be defined in VDM-SL. The type <code>(A1*…*An)</code> is the type composed of all tuples of values, the first element of which is from the type <code>A1</code> and the second from the type <code>A2</code> and so on. The composite or record type is a Cartesian product with labels for the fields. The type
<syntaxhighlight lang="text">
T :: f1:A1
     f2:A2
     ...
     fn:An
</syntaxhighlight>
is the Cartesian product with fields labelled <code>f1,…,fn</code>. An element of type <code>T</code> can be composed from its constituent parts by a constructor, written <code>mk_T</code>. Conversely, given an element of type <code>T</code>, the field names can be used to select the named component. For example, the type
<syntaxhighlight lang="text">
Date :: day:nat1
        month:nat1
        year:nat
inv mk_Date(d,m,y) == d<=31 and m<=12
</syntaxhighlight>
models a simple date type. The value <code>mk_Date(1,4,2001)</code> corresponds to 1 April 2001. Given a date <code>d</code>, the expression <code>d.month</code> is a natural number representing the month. Restrictions on days per month and leap years could be incorporated into the invariant if desired. Combining these:
<syntaxhighlight lang="text">
mk_Date(1,4,2001).month = 4
</syntaxhighlight>