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POCan traits in D be used for type classes?
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copied!<p>I'm new to D, and I'm looking for a good way to program with Haskell-like type classes e.g. Functors, Monoids, etc. in D.</p> <p>Is something like this implemented in Tango or Phobos? </p> <p>I've heard about traits which enable compile-time type checking for certain properties. Can they be used for type classes?</p> <p>I've tried a little bit with template specialization and come up with this:</p> <pre><code>// Monoid.d // generic Monoid gets called when there is no instance of Monoid for Type T class Monoid(T) { pragma(msg, "Type is not a Monoid"); } // Monoid instance for double class Monoid(T:double) { static T mzero() { return 0; } static T mappend(T a, T b ) { return a + b;} } // Monoid instance for int class Monoid(T:int) { static T mzero() { return 0; } static T mappend(T a, T b ) { return a + b;} } </code></pre> <p>A generic algorithm whose type parameter needs to be a Monoid could then be expressed as:</p> <pre><code>template genericfunctions() { T TestMonoid(T,N = Monoid!T)(T a) { return N.mappend(N.mzero(),a); } } </code></pre> <p>However, if you want to omit the template parameters, you have to import all needed Monoid instances and mixin the <code>genericfunctions</code> template.</p> <pre><code>import Monoid; import std.stdio; import std.conv; mixin genericfunctions; void main() { writefln(to!string(TestMonoid(3))); writefln(to!string(TestMonoid(3.3243))); } </code></pre> <p>You can now use ints and doubles as Monoids.</p> <p>However things get more complex when you have a type class like Functor whose instances are itself generic:</p> <pre><code>module Functors; // generic Functor like generic Monoid class Functor(alias T, A) { pragma(msg,"Not an instance of Functor"); } // very simple container to demonstrate functors behavior class FunctorTest(A) { public A a; this(A a) { this.a = a; } } // instance of Functor for FunctorTest!A class Functor(alias T:FunctorTest,A) { static T!B fmap(B)(T!A a, B delegate(A) fn) { return new T!B(fn(a.a)); } } </code></pre> <p>One algorithm would look like this:</p> <pre><code>template genericfunctions() { T TestMonoid(T,N = Monoid!T)(T a) { return N.mappend(N.mzero(),a); } // F is the Functor, A the functors type before, // B the functors Type after, N is the instance of Functor F!B fmap(alias F,A,B,N=Functor!(F,A))(F!A a, B delegate(A) fn) { return N.fmap!B(a,fn); } } </code></pre> <p>Luckily, you can omit the four template parameters when you use it:</p> <pre><code>mixin genericfunctions; void main() { auto a = new FunctorTest!int(3); auto b = fmap(a,(int b) {return b+ 0.5;}); writefln(to!string(b.a)); } </code></pre> <p>But when you want to use another Functor instance for the Type you have to specify all 4 type parameters of fmap. Is there a way in which you only need to specify the Instance and the other parameters could be deduced from this?</p> <p>Is there an alternative to the clumsy mixin workaround?</p> <p>Are there other disadvantages of this approach which I don't see? </p> <p>What about other ways?</p> <p>Thanks for reading this far and for taking the time to think and answer :)</p> <hr> <p><strong>Edit:</strong></p> <p>Is it possible to define constraints like the functor laws with unittest in D? That would be very nice.</p>

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