2 Why We Need Model-Based Testing

Systems with Complex State 165
Sequences appear in models where it is important to distinguish elements based on
order. For example, you could model a variable-length stack (LIFO) with operations
Push and Pop using a sequence:
static class Stack
{
static Sequence contents = Sequence.EmptySequence;
}
[Action]
static void Push(int value)
{
contents = contents.AddFirst(value);
}
static bool PopEnabled() { return !contents.IsEmpty; }
[Action]
static int Pop()
{
int result = contents.Head;
contents = contents.Tail;
return result;
}
Order matters in this example: Pop must produce the value given by the most
recent Push action.
Creating sequences
You can create a sequence by listing its elements as arguments to the constructor:
Sequence ratings = new Sequence("Poor", "Average",
"Excellent")
You can also start with the empty sequence and add elements to the beginning or
end programmatically:
Sequence squares = Map.EmptySequence;
for(int i = 0; i < 5; i += 1)
squares = squares.AddLast(i * i);
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