> data X a b = X a b<div>> instance Functor (X a) where</div><div>> fmap f (X a b) = X a (f b)</div><div><br></div><div>Yeah, that works just fine.</div><div><br></div><div><br></div><div><br><div class="gmail_quote">
On Fri, Jul 17, 2009 at 1:14 PM, Petr Pudlak <span dir="ltr"><<a href="mailto:deb@pudlak.name">deb@pudlak.name</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
Hi, I have probably a very simple question, but I wasn't able to figure it out<br>
myself.<br>
<br>
Consider a two-parameter data type:<br>
<br>
> data X a b = X a b<br>
<br>
If I want to make it a functor in the last type variable (b), I can just define<br>
<br>
> instance Functor (X a) where<br>
> fmap f (X a b) = X a (f b)<br>
<br>
But how do I write it if I want X to be a functor in its first type variable?<br>
Is that possible at all?<br>
Something like:<br>
<br>
> instance Functor ??? where<br>
> fmap f (X a b) = X (f a) b<br>
<br>
Thanks in advance,<br>
Petr<br>
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</blockquote></div><br></div>