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CHAPTER 5. PROGRAMMING CONCEPTS
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first or the last clause, the compiler will just treat this as another predicate
definition with the same name, but a different arity. Errors of this form are
very hard to spot.
Example
:-mode interface_type(+,+,-).
interface_type(_Node,local,local):!.
interface_type(Node,_Interface,backbone_net):node(Node,net),
!.
interface_type(Node,Interface,backbone):backbone_line(Node,Interface,_,_),
!.
interface_type(Node,Interface,backbone):backbone_line(_,_,Node,Interface),
!.
interface_type(Node,Interface,interconnection):group(interconnection,_,Node,Interface),
!.
interface_type(_Node,_Interface,customer).
Here we branch on information passed in the first two arguments, and return
a result in the last argument. The last clause is a default rule, saying that
the interface type is customer, if none of the other rules applied.
Some programmers perfer to make the output unification explicit, like so:
:-mode interface_type(+,+,-).
interface_type(_Node,local,Result):!,
Result = local.
interface_type(Node,_Interface,Result):node(Node,net),
!,
Result = backbone_net.
interface_type(Node,Interface,Result):backbone_line(Node,Interface,_,_),
!,
Result = backbone.
interface_type(Node,Interface,Result):backbone_line(_,_,Node,Interface),