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Edward Kmett | 14 Nov 02:20
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Re: A small (?) problem with type families

On Fri, Nov 13, 2009 at 3:36 PM, David Menendez <dave <at> zednenem.com> wrote:

On Fri, Nov 13, 2009 at 3:26 PM, Andy Gimblett <haskell <at> gimbo.org.uk> wrote:
> First a type family where the type Y is functionally dependent on
> the type X, and we have a function from Y to ().
>
>> class X a where
>>   type Y a
>>   enact :: Y a -> ()

This is ambiguous. Type families are not injective (that is, Y a ~ Y b
does not imply a ~ b), so there's no way for the compiler to figure
out which instance of X is being used when it encounters enact.

Note: that if you need this injectivity you can use a data family instead. 


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