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9. Flex Predicates 139<br />
<strong>flex</strong> toolkit<br />
AT = fail<br />
is_known( +Slot )<br />
The call succeeds only if there exists a current or default value for the Slot.<br />
The Slot should either be a compound term of the form<br />
@(Attribute,Frame) or it is the name of a global variable, in which case<br />
the name of the frame to be used is global.<br />
lookup( +Attribute,+Frame,-Value )<br />
Retrieve the current Value for an Attribute of a Frame as in<br />
isa_slot/3. Only if there is no current value will the default Value be<br />
returned instead (as in isa_default/3).<br />
If there is neither a current nor a default Value for the Attribute of the<br />
Frame, then the frame hierarchy is searched. The search finishes whenever<br />
an ancestor frame can be found which does have either a current or a<br />
default Value.<br />
The lookup/3 procedure is used whenever dereferencing <strong>flex</strong> objects.<br />
Examples<br />
?- lookup( colour, moby_dick, V ).<br />
V = white<br />
?- lookup( habitat, moby_dick, V ).<br />
V = [atlantic,arctic]<br />
?- lookup( habitat, whale, V ).<br />
V = land<br />
lookup( +Attribute,+Frame,-Value,-Ancestor )<br />
The lookup/4 procedure behaves as lookup/3 except that an additional<br />
piece of information, Ancestor, is returned. This indicates where exactly in<br />
the frame hierarchy the Value came from.<br />
Examples<br />
misfire( +Name )<br />
?- lookup( colour, moby_dick, V, A ).<br />
V = white, A = albino<br />
?- lookup( habitat, moby_dick, V, A ).<br />
V = [atlantic,arctic], A = moby_dick<br />
?- lookup( habitat, whale, V, A ).<br />
V = land, A = ocean_dweller
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