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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>[Redirecting to Haskell Cafe]<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Hi Greg<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I think it’s very likely that you *<b>can</b>* do what you
want. I have not taken long enough to understand just what you are
trying to do, but I do know why your program is failing, and I think you’ll
agree it *<b>should* </b>fail<b>. </b>Look at these lines (slightly
simplified)<o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>class CoreAgentSyntax x where<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> offer :: (CoreProcessSyntax p) => [n]
-> (p a x) -> x<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>data MinimalAgentSyntax nn pp = Concretion [nn] pp<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>instance CoreAgentSyntax (MinimalAgentSyntax nn pp) where<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> offer = Concretion <o:p></o:p></span></b></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The class decl says that every instance of ‘offer’
must have type<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> forall
p, forall n, forall a. (CoreProcessSyntax p) => [n] -> p a x -> x<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>where x is the instance type. Not the “forall n”!
Even once x is fixed, we must have a function that is polymorphic
in n,p,a<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>But in your instance declaration, offer = Concretion has the
type<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> [nn]
-> pp -> MinimalAgentSyntax nn pp<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>No for-alls, because the whole instance decl binds the ‘nn’.
Hence it’s just not polymorphic enough.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I’m not sure how to fix your problem. Perhaps you
need another argument for CoreAgentSyntax? <o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>class CoreAgentSyntax n x where<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> offer :: (CoreProcessSyntax p) => [n]
-> (p a x) -> x<o:p></o:p></span></b></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>instance CoreAgentSyntax nn (MinimalAgentSyntax nn pp) where<o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'> offer = Concretion <o:p></o:p></span></b></p>
<p class=MsoNormal><b><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></b></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>now you are in better shape. There’s another problem
because you use ‘p’ with kind * in some places and (*->*->*)
in others, but thats a separate issue. <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Oleg is good at this kind of stuff.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Simon<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> haskell-bounces@haskell.org
[mailto:haskell-bounces@haskell.org] <b>On Behalf Of </b>Greg Meredith<br>
<b>Sent:</b> 18 April 2007 14:53<br>
<b>To:</b> haskell@haskell.org<br>
<b>Subject:</b> [Haskell] haskell question<o:p></o:p></span></p>
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</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Haskellians,<br>
<br>
In the attached file i have implemented the bare bones core of the syntax --
pre structural equivalence -- of Milner's presentation of \pi via abstractions
and concretions. (There's a minor twist in that the notion of location affords
more structure than in Milner's original presentation which i have added to
support an ultimate probabilistic interpretation.) <br>
<br>
<span class=q>The file compiles with ghc as is. If you uncomment the last
section, however, you see that to close the loop on the constraints for the
agent syntax we hit a typing error. i <b><i>think</i></b> this is a hard
constraint in Haskell that prevents this level of generality in the
specification, but maybe you can see a way around it. i'm certainly no Haskell
expert. </span><br>
<br>
<span class=q>Best wishes,</span><br>
<br>
<span class=q>--greg</span><br>
<br>
P.S. The reason for this level of generality is that with it you can close the
loop and form a mutual recursion between the types for the syntax of processes
and the type of names -- without having that choice predesigned into the
structure of processes (or names). i can do this trick in OCaml with recursive
modules, but it's definitely not pretty. i was hoping Haskell could make it
pretty. <o:p></o:p></p>
<div>
<p class=MsoNormal><br clear=all>
<br>
<span class=e>-- </span><br>
<span class=e>L.G. Meredith</span><br>
<span class=e>Managing Partner</span><br>
<span class=e>Biosimilarity LLC</span><br>
<span class=e>505 N 72nd St</span><br>
<span class=e>Seattle, WA 98103</span><br>
<br>
<span class=e>+1 206.650.3740</span><br>
<br>
<span class=e><a href="http://biosimilarity.blogspot.com" target="_blank">http://biosimilarity.blogspot.com</a>
</span><o:p></o:p></p>
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