A First Fandango Spec

To create test inputs, Fandango needs a Fandango spec – a file that describes how the input should be structured.

A Fandango specification contains three types of elements:

• A grammar describing the syntax of the inputs to be generated;

• Optionally, constraints that specify additional properties; and

• Optionally, definitions of functions and constants within these constraints.

Only the first of these (the grammar) is actually required. Here is a very simple Fandango grammar that will get us started:

<start> ::= <digit>+
<digit> ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"

This grammar defines a sequence of digits:

• The first line in our grammar defines <start> – this is the input to be generated.

• What is <start>? This comes on the right-hand side of the “define” operator (::=).

• We see that <start> is defined as <digit>+, which means a non-empty sequence of <digit> symbols.

In Fandango grammars, you can append these operators to a symbol:

• + indicates one or more repetitions;

• * indicates zero or more repetitions; and

• ? indicates that the symbol is optional.

• What is a <digit>? This is defined in the next line: one of ten alternative strings, separated by |, each representing a single digit.

To produce inputs from the grammar, Fandango

• starts with the start symbol (<start>)

• repeatedly replaces symbols (in <...>) by one of their definitions (= one of the alternatives separated by |) on the right-hand side;

• until no symbols are left.

So,

• <start> first becomes <digit><digit><digit>... (any number of digits, but at least one);

• each <digit> becomes a digit from zero to nine;

• and in the end, we get a string such as 8347, 66, 2, or others.

Let us try this right away by invoking Fandango.