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<br><br><div class="gmail_quote">On Sat, Feb 5, 2011 at 12:25, Steven Keuchel <span dir="ltr"><<a href="mailto:steven.keuchel@gmail.com">steven.keuchel@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
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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</blockquote></div><br>