Yes, it's one things that data families do. Another is introducing *new* data types rather than reusing existing ones. - Conal<br><br><div class="gmail_quote">On Mon, Feb 14, 2011 at 1:41 PM, John Meacham <span dir="ltr"><<a href="mailto:john@repetae.net">john@repetae.net</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">Isn't this what data families (as opposed to type families) do?<br>
<br>
John<br>
<div><div></div><div class="h5"><br>
On Mon, Feb 14, 2011 at 1:28 PM, Conal Elliott <<a href="mailto:conal@conal.net">conal@conal.net</a>> wrote:<br>
> Is there a way to declare a type family to be injective?<br>
><br>
> I have<br>
><br>
>> data Z<br>
>> data S n<br>
><br>
>> type family n :+: m<br>
>> type instance Z :+: m = m<br>
>> type instance S n :+: m = S (n :+: m)<br>
><br>
> My intent is that (:+:) really is injective in each argument (holding the<br>
> other as fixed), but I don't know how to persuade GHC, leading to some<br>
> compilation errors like the following:<br>
><br>
> Couldn't match expected type `m :+: n'<br>
> against inferred type `m :+: n1'<br>
> NB: `:+:' is a type function, and may not be injective<br>
><br>
> I realize that someone could add more type instances for (:+:), breaking<br>
> injectivity.<br>
><br>
> Come to think of it, I don't know how GHC could even figure out that the two<br>
> instances above do not overlap on the right-hand sides.<br>
><br>
> Since this example is fairly common, I wonder: does anyone have a trick for<br>
> avoiding the injectivity issue?<br>
><br>
> Thanks, - Conal<br>
><br>
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><br>
><br>
</blockquote></div><br>