Editing Precondition (section)

==Example==
The [[factorial]] function is only defined where its parameter is an integer greater than or equal to zero.  So an implementation of the factorial function would have a precondition that its parameter be an integer ''and'' that the parameter be greater than or equal to zero. Alternatively the type system of the language may be used to specify that the parameter of the factorial function is a natural number (unsigned integer), which can be formally verified automatically by a compiler's type checker.

In addition where numeric types have a limited range (as they do in most programming languages) the precondition must also specify the maximum value that the parameter may have if overflow is not to occur. (e.g. if an implementation of factorial returns the result in a 64-bit unsigned integer then the parameter must be less than 21 because factorial(21) is larger than the maximum unsigned integer that can be stored in 64 bits). 
Where the language supports range sub-types (e.g. [[Ada (programming language)|Ada]]) such constraints can be automatically verified by the type system. More complex constraints can be formally verified interactively with a [[proof assistant]].