I would think the provided equalities could be constructed in a view pattern, possibly using unsafeCoerce. Dictionaries are harder to come by, but reflection might be an option. My two biggest gripes about pattern synonyms are really
1. The constraints for "constructor" application are forced to be much tighter than necessary. This is very sad because there doesn't seem to be anything inherently difficult about fixing it--just allow the user to specify the desired type signature for the synonym used as a constructor.
2. The exhaustivity check doesn't work yet.
David, Carter,
It would be nice to use pattern synonyms for this task but they do not
work quite as expected as they don't cause type refinement.
I quickly assembled this note to explain why.
http://mpickering.github.io/posts/2016-06-18-why-no-refinement.html
Matt
On Fri, May 27, 2016 at 4:50 AM, David Feuer <david.feuer@gmail.com> wrote:
> Scratch that. I think you might be right.
>
> On May 25, 2016 8:40 PM, "David Feuer" <david.feuer@gmail.com> wrote:
>>
>> Partially. Unfortunately, bidirectional pattern synonyms tie the types of
>> the pattern synonyms to the types of the smart constructors for no good
>> reason, making them (currently) inappropriate. But fixing that problem would
>> offer one way to this optimization, I think.
>>
>> On May 25, 2016 8:37 PM, "Carter Schonwald" <carter.schonwald@gmail.com>
>> wrote:
>>
>> could this be simulated/modeled with pattern synonyms?
>>
>> On Wed, May 25, 2016 at 7:51 PM, David Feuer <david.feuer@gmail.com>
>> wrote:
>>>
>>> I've started a wiki page,
>>> https://ghc.haskell.org/trac/ghc/wiki/OptimizeCountingGADTs , to consider
>>> optimizing GADTs that look like natural numbers but that possibly have
>>> "heavy zeros". Please take a look.
>>>
>>>
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>>> ghc-devs@haskell.org
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>>>
>>
>
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