Pattern synonym type flexibility

[Carter Schonwald]

Shouldn't the design simply be both directions are the dual of the other,
and pure in some sense ?

On Wednesday, April 20, 2016, David Feuer <david.feuer at gmail.com> wrote:

> To some degree, it probably could be. But I believe that imposing any
> substantial relationship between the smart constructor and the pattern
> synonym is likely to fall squarely into the category of things that are
> subtle, hard, and almost completely useless. In the arrangement I
> suggested, people would be free to do some things that "don't make sense",
> and that doesn't bother me in the least.
> On Apr 20, 2016 1:27 PM, "Carter Schonwald" <carter.schonwald at gmail.com
> <javascript:_e(%7B%7D,'cvml','carter.schonwald at gmail.com');>> wrote:
>
>> Would that duality be related to the given vs wanted constraints ?
>>
>> On Wednesday, April 20, 2016, David Feuer <david.feuer at gmail.com
>> <javascript:_e(%7B%7D,'cvml','david.feuer at gmail.com');>> wrote:
>>
>>> As far as I can tell from the 7.10 documentation, it's impossible to
>>> make a bidirectional pattern synonym used as a constructor have a
>>> different type signature than when used as a pattern. Has this been
>>> improved in 8.0? I really want something like
>>>
>>> class FastCons x xs | xs -> x where
>>>   fcons :: x -> xs -> xs
>>> class FastViewL x xs | xs -> x where
>>>   fviewl :: xs -> ViewL x xs
>>>
>>> pattern x :<| xs <- (fviewl -> ConsL x xs) where
>>>   x :<| xs = fcons x xs
>>>
>>> This would allow users to learn just *one* name, :<|, that they can
>>> use for sequences that are consable or viewable even if they may not
>>> be the other.
>>>
>>> If this is not yet possible, then I think the most intuitive approach
>>> is to sever the notions of "pattern synonym" and "smart constructor".
>>> So I'd write
>>>
>>> pattern x :<| xs <- (fviewl -> ConsL x xs)
>>> constructor (:<|) = fcons
>>>
>>> The current syntax could easily be desugared to produce *both* a
>>> pattern synonym and a smart constructor in the bidirectional case.
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>>
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