Re: 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  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?
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Re: Rewrapping with simple bidirectional pattern synonyms

[Edward Z. Yang]

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?
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Re: Rewrapping with simple bidirectional pattern synonyms

[Matthew Pickering]

The relevant ticket is - https://ghc.haskell.org/trac/ghc/ticket/12203



On Wed, Aug 24, 2016 at 11:11 PM, David Feuer  wrote:
> 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?
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Rewrapping with simple bidirectional pattern synonyms

[David Feuer]

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?
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