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.

For writing extended Fandango Specs, check out our Fandango language server and an IDE extension.

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 generating inputs with this right away by invoking Fandango.