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PORecursive data structures in haskell: prolog-like terms
 

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  1. PORecursive data structures in haskell: prolog-like terms
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    <p>I have a question about recursive data structures in Haskell (language that I'm currently trying to learn).</p> <p>I would like to encode in Haskell Prolog-like terms, but every solution I came up with has different drawbacks that I would really like to avoid. I would like to find a cheap and elegant way of encoding a BNF grammar in Haskell types, if you wish to see my problem from this perspective.</p> <p>Just as a reminder, some prolog terms could be <code>male</code>, <code>sum(2, 3.1, 5.1)</code>, <code>btree(btree(0, 1), Variable)</code>.</p> <h3>Solution 1</h3> <pre><code>data Term = SConst String | IConst Integer | FConst Double | Var String | Predicate {predName :: String, predArgs :: [Term]} </code></pre> <p>With this solution I can have nested predicates (since <code>predArgs</code> are <code>Term</code>), but I can't distinguish predicates from other terms in type signatures.</p> <h3>Solution 2</h3> <pre><code>data Term = SConst String | IConst Integer | FConst Double | Var String data Predicate = Predicate {predName :: String, predArgs ::[Either Term Predicate} </code></pre> <p>In this variant I can clearly distinguish predicates from basic terms, but the <code>Either</code> type in the <code>predArgs</code> list can be quite a nuisance to manage later in the code (I think... I'm new to Haskell).</p> <h3>Solution 3</h3> <pre><code>data Term = SConst String | IConst Integer | FConst Double | Var String | Struct String [Term] data Predicate = Predicate String [Term] </code></pre> <p>With this last solution, I split terms in two different types as before, but this time I avoid <code>Either Term Predicate</code> adding a <code>Struct</code> constructor in <code>Term</code> with basically the same semantics as <code>Predicate</code>.</p> <p>It's just like solution 1 with two predicate constructors for terms. One is recursion-enabled, <code>Struct</code>, and the other one, <code>Predicate</code> is to be able to distinguish between predicates and regular terms.</p> <p>The problem with this try is that <code>Struct</code> and <code>Predicate</code> are structurally equivalent and have almost the same meaning, but I will not be able to write functions that works - in example - both on <code>(Predicate "p" [])</code> and <code>(Struct "p" [])</code>.</p> <p>So again my question is: please, is there a better way to encode my predicates and terms such that:</p> <ol> <li>I'm able to distinguish between predicate and terms in type signatures;</li> <li>nested predicates like <code>p(q(1), r(q(3), q(4)))</code> are supported;</li> <li>I can write functions that will work uniformly on predicates, without any distinction like the one in solution #3?</li> </ol> <p>Please feel free to ask me for further clarifications should you need any.</p> <p>Thank you very much.</p>
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    1. COWhy do you want to be able to disinguish predicates and terms from their type signature? Is there any function that works on a predicate but not on a term? I would suggest you to google for "GADT" (generalized algebraic datatypes). They allow you to use method 1 while being able to carry information for the type system.
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    2. COYes, I want to model a simple intelligent Agent and it's beliefs will be encoded as predicates. I'll have a look at that GADT, thanks.
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    3. COGADT is a nice extension to the language, but it seems a little bit an overkill for my needs.
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