On 9/16/07, <b class="gmail_sendername">Mads Lindstrøm</b> <<a href="mailto:mads_lindstroem@yahoo.dk">mads_lindstroem@yahoo.dk</a>> wrote:<div><span class="gmail_quote"></span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hi all<br><br>If I have this type:<br><br> data Foo a b = ...<br><br>and this class<br><br> class Bar (x :: * -> *) where ...<br><br>I can imagine two ways to make Foo an instance of Bar. Either I must<br>"apply" the 'a' or the 'b' in (Foo a b). Otherwise it will not have the
<br>right kind. To "apply" the 'a' I can do:<br><br> instance Bar (Foo a) where ...<br><br>But what if I want to "apply" the 'b' ? How do I do that ?</blockquote><div><br>One easy way would be to create a newtype with the type parameters swapped:
<br><br> newtype Oof b a = Oof (Foo a b)<br> instance Bar (Oof b) where ...<br><br>Of course, if you want to partially apply the second parameter of a function, you use 'flip'. I thought for a while about whether there's some sort of typeclass hackery which is directly parallel to the use of 'flip', but couldn't come up with anything. Anyone?
<br><br>-Brent<br></div><br></div>