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

[Simon Peyton Jones]

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 <ghc-steering-committee-bounces at haskell.org> On Behalf Of Iavor Diatchki
Sent: 26 July 2018 09:09
To: ghc-steering-committee at 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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