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<p class="MsoNormal">Dear Haskell experts,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I working on a large Haskell project on Model Based Testing (see https://github.com/TorXakis/TorXakis)
<o:p></o:p></p>
<p class="MsoNormal">and I would like to get feedback on a design issue I have related to extension and parameterized classes.<o:p></o:p></p>
<p class="MsoNormal">I will describe a small simplified example to illustrate the issue<o:p></o:p></p>
<p class="MsoNormal">and I appreciate any feedback.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have a class that extends another class as follows:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">class Base a => Extend a where<o:p></o:p></p>
<p class="MsoNormal"> -- | Constructor from Base<o:p></o:p></p>
<p class="MsoNormal"> fromBase :: Base b => b -> a<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I can define an instance of Extend that hides the Base implementation as follows:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">{-# LANGUAGE ExistentialQuantification #-}<o:p></o:p></p>
<p class="MsoNormal">data BaseHidden = forall a . Base a => <o:p></o:p></p>
<p class="MsoNormal"> BaseHidden { _base :: a<o:p></o:p></p>
<p class="MsoNormal"> , ... -- additional extension info<o:p></o:p></p>
<p class="MsoNormal"> }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">instance Extend BaseHidden where<o:p></o:p></p>
<p class="MsoNormal"> fromBase b = BaseHidden b ... -- additional extension initialization<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I can define a parameterized data class of which the parameter is the type of the Base class as follows:<o:p></o:p></p>
<p class="MsoNormal">data BaseExpose a = BaseExpose { _base :: a<o:p></o:p></p>
<p class="MsoNormal"> , ... -- additional extension info<o:p></o:p></p>
<p class="MsoNormal"> }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">With this parameterized data class, I can make the following fromBase' function<o:p></o:p></p>
<p class="MsoNormal">fromBase' :: Base a => a -> BaseExpose a<o:p></o:p></p>
<p class="MsoNormal">fromBase' a = BaseExpose a ... -- additional extension initialization<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">My question is can I make BaseExpose an instance of Extend?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am aware that the signature of fromBase states that for all 'b' of Base<o:p></o:p></p>
<p class="MsoNormal">an Extend of type 'a' must be able to be made.<o:p></o:p></p>
<p class="MsoNormal">Yet, in my case 'a' is a parameterized class 'BaseExpose x' and
<o:p></o:p></p>
<p class="MsoNormal">since the type of Extend is provided (it is a constructor)<o:p></o:p></p>
<p class="MsoNormal">I think it is acceptable that 'BaseExpose b' is returned as type.<o:p></o:p></p>
<p class="MsoNormal">I have looked at some extensions, such as InstanceSigs, yet I found no solution.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Does anybody on this mailing list have an answer or solution for me?<o:p></o:p></p>
<p class="MsoNormal">Also experience in a large project related to maintenance for the two alternatives (hiding or exposing types) is appreciated!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="NL">Thanks in advance,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="NL"> Pierre van de Laar<o:p></o:p></span></p>
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