Deep fmap with GADTs and type families.

Manuel M T Chakravarty chak at
Fri Mar 6 00:11:17 EST 2009

David Menendez:
> On Thu, Mar 5, 2009 at 10:07 PM, Dan Doel <dan.doel at> wrote:
>> But we've so far not been able to find a way of merely annotating  
>> the original
>> into working. So, I was wondering if any of the more knowledgeable  
>> folks here
>> could illuminate what's going wrong here, and whether I should  
>> expect my
>> original code to work or not.
> I'll bet the problem has to do with the fact that "f" only appears in
> "Nest n f a", so the type checker can't figure out what "f" is.

Exactly.  In other words, the signature is ambiguous.


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