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POEmulating Prolog backtracking in F#
 

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    <p>I am currently involved in a project to develop an application able to consider a set of nodes and connections and find the shortest path (a common and well-known issue) between two nodes (on allowed connections). Well I don't really have to build an application from zero, but just need to "convert" a Prolog pre-existing application in f#. I thought I bit about it and finally asked myself one question: "Instead of developing a special purpose solution and implementing new algorithms specifically binded to this problem, can I create a program able to accept facts like Prolog and use them to make queries or something similar?".</p> <p>By doing so I would create a set of facts (like in Prolog) and then use them to make queries. So, considering now this new issue (converting Prolog in F#) I need to find a way to create facts like these:</p> <pre><code>myfact1(el1, el2,..., eln). myfact2(el1, el2,..., elm). ... myfactk(el1, el2,..., elp). </code></pre> <p>to something in a similar syntax like:</p> <pre><code>fact factname1: el1 el2 ... eln; fact factname2: el1 el2 ... elm; ... fact factnamek: el1 el2 ... elp; </code></pre> <p>I know that F# is very well for parsing, so I think that parsing this would probably not be a problem.</p> <p>OK! Now that it is parsed, I should define an algorithm that, while parsing the code, stores all facts in some sort of knowledge (nothing more than a table). In order to make then all needed associations.</p> <p>For example a solution might be a hashtable that considers all associations</p> <pre><code>factname1 -&gt; el1 factname1 -&gt; el2 ... factname1 -&gt; eln factname2 -&gt; el1 factnale2 -&gt; el2 ... factname2 -&gt; elm factname3 -&gt; el1 ... ... factnamek -&gt; el1 factnamek -&gt; el2 ... factnamek -&gt; elp </code></pre> <p>By doing so I will always be able to solve queries. For example consider the following Prolog fact</p> <pre><code>mother(A, B) % This means that A is mother of B mother(C, D) </code></pre> <p>In F# I create</p> <p>fact mother: A B; fact mother: C D;</p> <p>My hashtable is:</p> <p>mother -> A | B mother -> C | D</p> <p>The first col is the fact name and the second is the value (here a tuple).</p> <p>If I want to search: "who is the mother of B" --> I search for mother, and look for value, I find B, I look in the tuple and discover A!</p> <p>Well it seems working. But facts are easy to implement. What about rules? For example rule parents:</p> <pre><code>parents(A, B, C) :- mother(A, C), father (B, C) </code></pre> <p>In F# using my syntax? Well I came up with this idea:</p> <pre><code>rule parents: A, B, C =&gt; mother A C and father B C </code></pre> <p>When my parser encounters a rule, what should it do? I would like to create some sort of record in a table like I did and be able, later, to make queries in order to specify a subject and get its parents or specify a father and get all sons and so on... What would you do?</p>
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