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POType parameters versus member types in Scala
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  1. POType parameters versus member types in Scala
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    copied!<p>I'd like to know how do the member types work in Scala, and how should I associate types.</p> <p>One approach is to make the associated type a type parameter. The advantages of this approach is that I can prescribe the variance of the type, and I can be sure that a subtype doesn't change the type. The disadvantages are, that I cannot infer the type parameter from the type in a function.</p> <p>The second approach is to make the associated type a member of the second type, which has the problem that I can't prescribe bounds on the subtypes' associated types and therefore, I can't use the type in function parameters (when x : X, X#T might not be in any relation with x.T)</p> <p>A concrete example would be:</p> <p>I have a trait for DFAs (could be without the type parameter)</p> <pre><code>trait DFA[S] { /* S is the type of the symbols in the alphabet */ trait State { def next(x : S); } /* final type Sigma = S */ } </code></pre> <p>and I want to create a function for running this DFA over an input sequence, and I want</p> <ul> <li>the function must take anything <code>&lt;% Seq[alphabet-type-of-the-dfa]</code> as input sequence type</li> <li>the function caller needn't specify the type parameters, all must be inferred</li> <li>I'd like the function to be called with the concrete DFA type (but if there is a solution where the function would not have a type parameter for the DFA, it's OK)</li> <li>the alphabet types must be unconstrained (ie. there must be a DFA for Char as well as for a yet unknown user-defined class)</li> <li>the DFAs with different alphabet types are not subtypes</li> </ul> <p>I tried this:</p> <pre><code>def runDFA[S, D &lt;: DFA[S], SQ &lt;% Seq[S]](d : D)(seq : SQ) = .... </code></pre> <p>this works, except the type S is not inferred here, so I have to write the whole type parameter list on each call site.</p> <pre><code>def runDFA[D &lt;: DFA[S] forSome { type S }, SQ &lt;% Seq[D#Sigma]]( ... same as above </code></pre> <p>this didn't work (invalid circular reference to type D??? (what is it?))</p> <p>I also deleted the type parameter, created an abstract type Sigma and tried binding that type in the concrete classes. runDFA would look like</p> <pre><code>def runDFA[D &lt;: DFA, SQ &lt;% Seq[D#Sigma]]( ... same as above </code></pre> <p>but this inevitably runs into problems like "type mismatch: expected <code>dfa.Sigma</code>, got <code>D#Sigma</code>"</p> <p>Any ideas? Pointers?</p> <p>Edit:</p> <p>As the answers indicate there is no simple way of doing this, could somebody elaborate more on why is it impossible and what would have to be changed so it worked?</p> <p>The reasons I want runDFA ro be a free function (not a method) is that I want other similar functions, like automaton minimization, regular language operations, NFA-to-DFA conversions, language factorization etc. and having all of this inside one class is just against almost any principle of OO design.</p>
 

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