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nil

The constant nil represents an undefined value. Formally speaking, nil belongs to no type. It is a symbol external to the RDF universe of discourse that is employed in Fabl to represent the absence of a value. For example, nil is returned by x.p if the resource x does not possess a value for the property p.

nil has Resource as its nominal type, and can be cast to any type except int, boolean, and double.

The function

boolean function nul(<Type> x)

determines whether x is a nul value. nul applies to all types except int, boolean, and double.

nil is not the same as the resource rdf:nil. The former indicates absence of an RDF resource, while the latter is a legitimate RDF resource used to indicate the end of an rdf:List.