Allow label in pattern matching?

[Wizek]

I've been reading the Example.hs file, and want to share some quick thoughts. This is a low priority ticket on my part, feel free to only respond to this days or weeks from now at your leisure. I'm writing now because I don't want to forget about it.

So, reading this section:

get2D'' :: (r ≈ "x" .== Double .+ "y" .== Double, Disjoint r rest)
        => Rec (r .+ rest)
        -> Rec r
get2D'' ((Label :: Label "x") :== n1 :+ (Label :: Label "y") :== n2 :+ _)
          = #x .== n1 .+ #y .== n2

It would indeed be much nicer if we could just write:

get2D'' :: (r ≈ "x" .== Double .+ "y" .== Double, Disjoint r rest)
        => Rec (r .+ rest)
        -> Rec r
get2D'' (#x :== n1 :+ #y :== n2 :+ _)
          = #x .== n1 .+ #y .== n2

Trying to do this now, we get:

> :t \(#a) -> 1
<interactive>:1:3: error: Parse error in pattern: #a

A parse error. So, it might be quite trivial to fix this upstream in GHC's parser, just make #a de-sugar to (Label :: Label "a") in a pattern context as well. And I am guessing/hoping/wondering that GHC folks would be very happy to take this contribution. I've briefly looked if there may be a trac ticket even open about this, but I haven't found any yet. So maybe the only reason this isn't working is because no one was inconvenienced by it enough to fix it or raise it.

What do you think?

[dwincort]

It would be great if this were implemented. There is https://ghc.haskell.org/trac/ghc/ticket/13116, but no work has been done on it.