Cool. It's more and more clear guys.<br>Thanks a lot. I'll check that "expression problem".<br>"Existential types" sounds a bit scary :)<br><br><br><div class="gmail_quote">On Tue, Sep 15, 2009 at 3:26 PM, Sean Leather <span dir="ltr"><<a href="mailto:leather@cs.uu.nl">leather@cs.uu.nl</a>></span> wrote:<br>
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Perimeter doesn't make sense for Sphere or Cylinder. So we could define a type class for objects that have perimeter and make an instance of it only for Circle (data Circle = Circle Position Radius). Make sense. But these three functions above have desired behaviour. If user has a list of objects like [Sphere, Circle, Circle, Cylinder] he would like to calculate perimeters of each object using map perimerer list (in this case we also have to modify Geometry data type).<br>
So we could make instances of "perimeter" type class for all objects and return zero in case if perimeter doesn't make sense.<br>Same as previous version but with typeclasses and with additional constructors (constructors for each type of object + constructors in Geometry data). Looks a bit overcomplicated.<br>
Any reasons to use type classes in this case? Maybe there is something I'm missing?<br></blockquote></div><div><br>If you're talking about a single datatype with multiple constructors, then the function 'perimeter :: Geometry -> Maybe Double' makes sense. If you're talking about multiple datatypes, then you probably want to go type class route.<br>
<br>data Sphere = Sphere ...<br>data Circle = Circle ...<br><br>class Perimeter a where perimeter :: a -> Double<br>instance Perimeter Circle where perimeter (Circle ...) = ...<br>-- No instance for Sphere<br><br>class Volume a where volume :: a -> Double<br>
instance Volume Sphere where volume (Sphere ...) = ...<br>-- No instance for Circle<br><br>You have to decide whether (1) a datatype Geometry makes sense or (2) a datatype per geometric entity is better. One advantage to #1 is that writing functions over the datatype is easy. One advantage to #2 is that you have fewer (partial) 'Maybe' functions. This is also related to the "expression problem," a Googleable term.<br>
<br>As for having a list of objects, you can do it with either approach. The second approach may require existential types.<br><br>Regards,<br>Sean<br></div></div>
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