Re: Pattern synonym type flexibility

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 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" javascript:_e(%7B%7D,'cvml','carter.schonwald@gmail.com');> wrote:

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Would that duality be related to the given vs wanted constraints ?

On Wednesday, April 20, 2016, David Feuer javascript:_e(%7B%7D,'cvml','david.feuer@gmail.com');> wrote:

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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. _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs