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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">It’s certainly true that, with the current setup, a monomorphic instance will look like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">instance Typeable Foo where<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> typeRep# = \_ -> (...something...) :: TypeRep<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">and the optimiser will float the (...something...) to top level. But this is an optimisation, not true by construction. For example,
a non-top-level instance might look like<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">instance (Typeable a, Typeable b) => Typeable (a b) where</span><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="Code"><span lang="EN-US" style="mso-fareast-language:EN-US"> typeRep</span><span style="mso-fareast-language:EN-US"># = \_ -> mkAppTy (typeRep# (undefined::Proxy a))<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> (typeRep# (undefined :: Proxy b))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">And that in turn will become<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">$dfTyApp = /\a b. \(d1::Typeable a) (d2::Typeable b).<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> \_ -> mkAppTy (d1 a) (d2 b)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">(I’m missing out some newtypes etc.) Now, will the (d1 a) and (d2 b) be floated outside the \_? Not so certain. It depends on
the let-floater.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">The good thing about the Tagged stuff is that there is no lambda in the first place, so the issue doesn’t arise.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">It’s ok if the programming interface is less easy to use; Data.Typeable.typeOf is already a function, not a method. The method is
called typeRep#, and is internal.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","sans-serif";color:#1F497D;mso-fareast-language:EN-US">So I think you can go ahead and improve the design. But it’s clearly a library-committee decision; I’ll just do what you say.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","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:"Verdana","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:"Verdana","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>Dan Doel<br>
<b>Sent:</b> 10 March 2014 18:02<br>
<b>To:</b> Roman Cheplyaka<br>
<b>Cc:</b> Haskell Libraries<br>
<b>Subject:</b> Re: Data.Dynamic: Any vs existential<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">
On Mon, Mar 10, 2014 at 6:35 AM, Roman Cheplyaka <<a href="mailto:roma@ro-che.info" target="_blank">roma@ro-che.info</a>> wrote:<o:p></o:p></p>
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Ok, one reason is that the TypeRep won't be cached in the Dynamic value.<br>
Even in GHC 7.8 Typeable is defined as<br>
<br>
class Typeable a where<br>
typeRep# :: Proxy# a -> TypeRep<br>
<br>
instead of<br>
<br>
class Typeable a where<br>
typeRep :: Tagged a TypeRep<br>
<br>
Why? Is this an oversight?<o:p></o:p></p>
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<o:p> </o:p></p>
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<span style="font-family:"Arial","sans-serif"">I talked with Ed about this, and he noted that this might be a false problem. Certainly, the Proxy# is never actually used, so as long as the generated instances are of the form:<o:p></o:p></span></p>
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<span style="font-family:"Arial","sans-serif""> typeRep# = let tr = ... in \_ -> tr<br>
<br>
the TypeReps are shared, and are at most a bit of indirection away. Also, how different are (Tagged tr :: Tagged a TypeRep) and ((\_ -> tr) :: Proxy# a -> TypeRep) at a low level? I know the constructor of the former disappears in core, but don't know what
specifically happens with 0-width fields like Proxy#. Is the latter slightly less efficient? If so, can the optimizer eliminate the difference for cases like this?<o:p></o:p></span></p>
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<span style="font-family:"Arial","sans-serif"">-- Dan<o:p></o:p></span></p>
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