By Raymond Turner
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Extra resources for Constructive foundations for functional languages
8 Diagrammatic representation of backtracking It is interesting to note that the search mechanism of Prolog is a type of search known as a depth first search. 6) are elicited, we fmd that we are searching a tree of the form shown below. GOAL NODE Node 1 = supervises (X,V):-manager (X), clerk (V). Node 2 = supervises (X,V):- clerk (X), typist (V). Node 3 = supervises (X,V):-manager (X), typist (VM. Node 4 = manager (patel). Node 5 = clerk (jones). Node 6 = clerk (smith). A node is shown by the * symbol.
Or "who supervises who"? This is a slightly more complex example, so we will number the steps which are taken to generate solutions. (1) The predicate is supervises and the first clause to match with the correct predicate and the correct number of arguments is the rule supervises (X, Y) :- manager (X), clerk (Y). The first sub-goal manager (X). then needs to be evaluated in order to establish the truth or otherwise of the goal. (2) Prolog now attempts to find the first clause that matches with manager (X).
If we now ask the question then the search tree will be as shown. Notice how the cut prevents the search returning to node 3 and hence effectively prunes the tree that would be produced were the cut not present (the pruned portion is shown by broken lines). - find _a_ supervisor (X,V). ,. J/! ""'~_m, I I I *I I I * * I * I I I / *\ I I I \ \ *I I I * I * I I I I I • / I I *\ \ \ * \ I I I I At this stage we have introduced the cut symbol and applied it to a simple example in order to demonstrate the concept of using the cut to restrict backtracking.