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POVisitor Design Pattern in OCaml
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copied!<p>I am attempting to implement the Visitor Design Pattern using OCaml's OO constructs and type system and am running into problems upon instantiation of an Element.</p> <pre><code>class virtual ['hrRep] employee = object method virtual receiveEvaluation : 'hrRep -> unit method virtual getName : string end;; class ['hrRep] accountant myName = object (self : 'a) inherit ['hrRep]employee val name = myName method receiveEvaluation rep = rep#visitAccountant self method getName = name end;; class ['hrRep] salesman myName = object (self : 'a) inherit ['hrRep]employee val name = myName method receiveEvaluation rep = rep#visitSalesman self method getName = name end;; class virtual ['accountant, 'salesman] hrRep = object (self) method virtual visitSalesman : 'salesman -> unit method virtual visitAccountant : 'accountant -> unit end;; class ['employee, 'salesman] lowerLevelHRRep = object (self) inherit ['employee, 'salesman]hrRep method visitSalesman s = print_endline ("Visiting salesman "^s#getName) method visitAccountant a = print_endline ("Visiting accountant "^a#getName) end;; let s1 : (<visitSalesman : 'a -> unit>) salesman = new salesman "Bob";; let a1 : (<visitAccountant : 'a -> unit>) accountant = new accountant "Mary";; let s2 : (<visitSalesman : 'a -> unit>) salesman = new salesman "Sue";; let h1 : (<getName : string>, <getName : string>) lowerLevelHRRep = new lowerLevelHRRep;; s1#receiveEvaluation h1;; </code></pre> <p>The error I get upon compilation is:</p> <pre><code>The type of this expression, <visitSalesman : 'a -> unit; _.. > salesman as 'a, contains type variables that cannot be generalized. </code></pre> <p>However, the code compiles minus the line instantiating the <code>salesman</code>. </p> <p>How do I go about instantiating the <code>salesman</code> while maintaining the classes' functionality ?</p> <p><strong>Edit</strong> Error received with call to receiveEvaluation: </p> <pre><code>This expression has type (<getName:string>, < getName:string>) lowerLevelHRRep but is here used with type <visitSalesman : 'a salesman -> unit > as 'a. </code></pre> <p>The second object type has no method <code>visitAccountant</code>.</p>

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