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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Can you give an example of what you’d like to write, but can’t?<br>
<br>
Simon<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""> glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>José Pedro Magalhães<br>
<b>Sent:</b> 04 April 2011 10:53<br>
<b>To:</b> GHC users<br>
<b>Cc:</b> Steven Keuchel<br>
<b>Subject:</b> Re: [Haskell] Polymorphic types in RHS of type instances<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Hi,<br>
<br>
[Moving to <a href="mailto:glasgow-haskell-users@haskell.org">glasgow-haskell-users@haskell.org</a>]<br>
<br>
I would also like to know the answer to this question. While I can imagine it has something to do with type checking/inference, it is not immediately clear to me where the problem lies.<br>
<br>
<br>
Thanks,<br>
Pedro<o:p></o:p></p>
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<p class="MsoNormal">On Sat, Feb 5, 2011 at 12:25, Steven Keuchel <<a href="mailto:steven.keuchel@gmail.com">steven.keuchel@gmail.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Hi list,<br>
<br>
I was wondering why GHC doesn't allow usage of polymorphic types in<br>
the right-hand side of type instance declarations for type families.<br>
The GHC user guide states: "The right-hand side of a type instance<br>
must be a monotype (i.e., it may not include foralls) [...]", but it<br>
doesn't state the reason.<br>
<br>
I stumbled upon this limitation when I was trying to generically<br>
calculate Johann's and Ghani's interpreter (transformers) for nested<br>
data types from their "Initial Algebra Semantics is Enough!" paper.<br>
<br>
Cheers,<br>
Steven<br>
<br>
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