Re: [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)

As a co-author of the paper that came from this: yes, please! :)

On Jul 26, 2018, at 6:17 AM, Simon Peyton Jones via ghc-steering-committee wrote:

I’m a strong supporter. I think the proposal itself (which will have longevity) could be improved significantly, and have commented to that effect.

Simon

From: ghc-steering-committee On Behalf Of Iavor Diatchki Sent: 26 July 2018 09:09 To: ghc-steering-committee@haskell.org Subject: [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)

Hello,

let's also start the discussion on feature request 126. The idea here is that we allow the @ notation for explicit type applications to also be used on constructors in patterns. Using @ with a constructor in a pattern has the same meaning as it does it an expression: the provided type is used to instantiate the corresponding type parameter of the constructor. If the type contains variables, those are treated in the same way as in #128, where "unbound" variables name the matching types. Here are some examples:

f1 (Just @Int x) = x -- This has type `Maybe Int -> Int`

f2 (Just @[a] x) = x == "c" -- `a` is an alias for `Char`

f3 (SomeException @e ex) = ... -- `e` is a name for the existentially hidden exception type

Overall I think that is a simple and natural extension to the way @ already works, and I propose that we accept it.

Thoughts?

-Iavor

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