Rewrapping with simple bidirectional pattern synonyms

[David Feuer]

I'm glad to see I'm not the only one who wants this!

On Wed, Aug 24, 2016 at 6:13 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> I filed a ticket for precisely this:
>
>     https://ghc.haskell.org/trac/ghc/ticket/12203
>
> Edward
>
> Excerpts from David Feuer's message of 2016-08-24 18:11:16 -0400:
>> I'm trying to write a bidirectional pattern synonym which is morally
>> simple, but as far as I can tell there's no way to write it as a
>> simple bidirectional pattern synonym.
>>
>> What I want to say is
>>
>> pattern TSnoc (TSnocList xs) x = TSnocList (CL.TCons (Dual x) xs)
>>
>> (where TSnocList and Dual are both newtype constructors)
>>
>> But I don't see a way to do it without being explicitly bidirectional:
>>
>> pattern TSnoc xs x <- (tsViewR -> ViewR xs x) where
>>   TSnoc (TSnocList xs) x = TSnocList (CL.TCons (Dual x) xs)
>>
>> tsViewR :: TSnocList c x y -> ViewR c (TSnocList c) x y
>> tsViewR (TSnocList CL.TNil) = EmptyR
>> tsViewR (TSnocList (CL.TCons (Dual x) xs)) = ViewR (TSnocList xs) x
>>
>> Would it be possible to make this simple thing simple?