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<p class="MsoNormal">> {-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances, GADTs #-}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">lets say I have a data type<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> data S a = S a deriving Show<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">and a nice simple typeclass<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> class Foo a where<o:p></o:p></p>
<p class="MsoNormal">> inc :: a -> S a<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">make Integer an instance and....<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> instance Foo Integer where<o:p></o:p></p>
<p class="MsoNormal">> inc x = S x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">brilliant...<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">now lets say I want my typeclass to enforce that any result of inc is also of a type that is an instance of the typeclass<o:p></o:p></p>
<p class="MsoNormal">I'll follow my nose...which causes me a problem that makes me realise I maybe I don't understand whats going on.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> class Bar a where<o:p></o:p></p>
<p class="MsoNormal">> inc' :: (Bar (S a)) => a -> S a<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">follow it to....<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> instance Bar Integer where<o:p></o:p></p>
<p class="MsoNormal">> inc' x = S x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">this WORKS!...even though "S Integer" is not an instance of Bar!...maybe I don't understand.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> -- x = inc' (1 :: Integer)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This would go BOOM ....”no instance of (Bar (S Integer))"<o:p></o:p></p>
<p class="MsoNormal">...so that makes sense....<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">How do I force Haskell to check the constraint before I evaluate an expression?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">follow my nose?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> -- class (Baz (S a)) => Baz a where<o:p></o:p></p>
<p class="MsoNormal">> -- inc'' :: a -> S a<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">BOOM ...."Cycle in class declaration"....<o:p></o:p></p>
<p class="MsoNormal">booo..but I suppose thats understandable....<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">so<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">a) is my use of typeclasses correct?...or am I missing something?<o:p></o:p></p>
<p class="MsoNormal">b) if so, how do I force the compiler to make sure my typeclass operations is nicely closed (under the typeclass)?<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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