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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">We have a constraint (a~b) that can appear in a type signature<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> f :: (a~b) => a -> b
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">We can also have a type-level equality function<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> type family Eq a b where<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> Eq a a = True<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> Eq a b = False<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">The type-level Boolean reflects the (a~b) constraint precisely.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">You sound as if you want a type family Satisfied, so you
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> type family Satisfied :: Constraint -> Bool<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Then you could have<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> instance Ord T where …<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> type instance Satisfied (Ord T) = True<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">The difficulty is that we can never return False, because you can always make T an instance of Ord “later”. And you can’t have Satisfied
(Ord T) returning True in one place and Falsie in another. It can get stuck (not reduce) but it can’t return False.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I don’t know if this would be useful to you.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">And for more complex instances lie<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> instance Ord a => Ord [a]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I don’t know what you want to say for<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"> type instance Satisfied (Ord a) = ???<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif""> Libraries [mailto:libraries-bounces@haskell.org]
<b>On Behalf Of </b>Ryan Newton<br>
<b>Sent:</b> 15 October 2013 04:03<br>
<b>To:</b> Pedro Magalhães (dreixel@gmail.com)<br>
<b>Cc:</b> Haskell Libraries; Andres Löh; Ganesh Sittampalam; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Proposal: GHC.Generics marked UNSAFE for SafeHaskell<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hey, that's an awesome formulation! Thanks Pedro.<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Any idea how much work this would be to implement in GHC, if it did garner approval?<o:p></o:p></p>
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<o:p> </o:p></p>
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On Tue, Oct 8, 2013 at 3:48 AM, José Pedro Magalhães <<a href="mailto:dreixel@gmail.com" target="_blank">dreixel@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hi,<o:p></o:p></p>
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<o:p> </o:p></p>
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On Mon, Oct 7, 2013 at 10:32 AM, Dag Odenhall <<a href="mailto:dag.odenhall@gmail.com" target="_blank">dag.odenhall@gmail.com</a>> wrote:<o:p></o:p></p>
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<p style="margin:0px!important">Here‘s a thought: doesn’t <code><span style="font-size:10.0pt;font-family:Consolas;border:solid #EAEAEA 1.0pt;padding:0cm;background:#F8F8F8">Generic</span></code> already have an unused phantom type that's only there “just in
case we need to add something in the future”?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">No, it doesn't.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Just a thought: what if we had a type family<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Courier New"">type family Derives (t :: k1) (c :: k2) :: Bool</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">which would automatically be instantiated by GHC appropriately? E.g., if the user had the following code:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">data MyData = MyData deriving (Eq, Generic)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">deriving instance Show MyData</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Courier New"">instance Ord MyData</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">GHC would automatically instantiate:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Courier New"">type instance Derives MyData Eq = True<br>
type instance Derives MyData Generic = True<br>
type instance Derives MyData Show = True<br>
type instance Derives MyData Ord = False</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Would this be something Ryan could use for detecting safe instances for LVish?<br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal">Cheers,<br>
Pedro<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
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<p class="MsoNormal"><o:p> </o:p></p>
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