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<p class="MsoNormal">Hello,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I largely don’t know what I’m doing or even trying to do, it is a voyage into the unknown….but….if I go…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> {-# LANGUAGE MultiParamTypeClasses #-}<o:p></o:p></p>
<p class="MsoNormal">> {-# LANGUAGE FunctionalDependencies #-}<o:p></o:p></p>
<p class="MsoNormal">> {-# LANGUAGE FlexibleInstances #-}<o:p></o:p></p>
<p class="MsoNormal">> {-# LANGUAGE UndecidableInstances #-}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> class Foo x y | x -> y, y -> x<o:p></o:p></p>
<p class="MsoNormal">> instance Foo Integer Integer<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">That seems to work….and my head seems to say…your created some sort of binary relation between 2 types…and made <Integer,Integer> a member of it…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Something like that anyway….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Then I go….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> data Bar<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> instance Foo Bar x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Error!....but I’m think I understand this….I can’t claim that <Bar,x> is a member of Foo and <Integer,Integer> is member of Foo and preserve my functional dependencies, because <Bar,Integer> is now a member of Foo..<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Bad programmer…….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So how I naively go….<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> class NotAnInteger a<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> instance (NotAnInteger x) => Foo Bar x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I haven’t declared integer to be “NotAnInteger”….so (in a closed world)….this would seem to exclude the contradiction….but…<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:Consolas;color:#AF0000">Functional dependencies conflict between instance declarations:<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:Consolas;color:#AF0000"> instance Foo Integer Integer -- Defined at liam1.lhs:7:12<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:Consolas;color:#AF0000"> instance NotAnInteger x => Foo Bar x -- Defined at liam1.lhs:13:12<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So <o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:54.0pt;text-indent:-36.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="mso-list:Ignore">i)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>I clearly don’t understand something about the type system.<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:54.0pt;text-indent:-36.0pt;mso-list:l0 level1 lfo1">
<![if !supportLists]><span style="mso-list:Ignore">ii)<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>I don’t know how to restrict type variables in instance declarations….i.e. how do I use the notion of “Foo” across different combinations of types, without them colliding.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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