Pattern synonym type flexibility
carter.schonwald at gmail.com
Wed Apr 20 17:27:08 UTC 2016
Would that duality be related to the given vs wanted constraints ?
On Wednesday, April 20, 2016, David Feuer <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.
> ghc-devs mailing list
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