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.
Would that duality be related to the given vs wanted constraints ?
On Wednesday, April 20, 2016, David Feuer <david.feuer@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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