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POParser generator, predicate not returing true or false
 

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  1. POParser generator, predicate not returing true or false
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    <p>I have a some problem with my PROLOG assignment.</p> <p>Namely, I have to write predicate accept(+Automaton, +Word) that return true if Word belong to language (Automaton is represented by Goto table, Action table and grammar, but it is not so important), false otherwise (it is required in my assignment to return true or false). I have written</p> <pre><code>accept(Automaton, Word) :- parse(Automaton, Word, RightmostDeriv), nl, print_list(RightmostDeriv), nl. </code></pre> <p>where</p> <p>Predicate <code>parse(+Automaton, +Word, -RightmostDeriv)</code> calculate righmost derivation. If Word cannot be parsed then <code>RighmostDeriv == ['mistake']</code></p> <p>As you see I can print rigthmost derivation, but I do not know how to return false or true.</p> <p>I have found this <a href="https://csc.cdf.toronto.edu/mybb/showthread.php?tid=2100" rel="nofollow">https://csc.cdf.toronto.edu/mybb/showthread.php?tid=2100</a> and I see that here can be a problem. </p> <p>But maybe is here any solution (e.g with member predicate, something like member('mistake', RightmostDeriv) ) to return true or false?</p> <p>Edit: having</p> <pre><code>accept(Automaton, Word) :- parse(Automaton, Word, RightmostDeriv), nl, print(RightmostDeriv), nl. </code></pre> <p>I received (Test 2 - example from wikipedia <a href="http://en.wikipedia.org/wiki/LR_parser#Additional_Example_1.2B1" rel="nofollow">http://en.wikipedia.org/wiki/LR_parser#Additional_Example_1.2B1</a>)</p> <pre><code>[4] ?- make. =============== Test 1 =================== [mistake] =============== Test 2 =================== [5,3,5,2] true . [4] ?- </code></pre> <p>and having:</p> <pre><code>accept(Automaton, Word) :- parse(Automaton, Word, RightmostDeriv), RightmostDeriv \= ['mistake']. </code></pre> <p>I received (infinite loop)</p> <pre><code>?- make. =============== Test 1 =================== ^CAction (h for help) ? break % Break level 1 [1] ?- </code></pre>
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    1. USJosephConrad
    1. USJosephConrad
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    1. COIf I understood correctly, you'd have to add `RighmostDeriv \= ['mistake']` to check whether the word was accepted or not, yielding true or false as a result of the query.
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      1. POParser generator, predicate not returing true or false
      1. USgusbro
    2. COhmm.. it does not help.I have enclosed in the edited part of question result of query.
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      1. POParser generator, predicate not returing true or false
      1. USJosephConrad
    3. COIf it does not end the problem is that you are leaving choicepoints somewhere, allowing the prolog engine to backtrack indefinitely. Looking at your example linked from wikipedia you can see that the first two rules may loop forever...
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      1. POParser generator, predicate not returing true or false
      1. USgusbro
 

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