[ghc-steering-committee] Discussion about "Type Application in Patterns" (#126)
Richard Eisenberg
rae at cs.brynmawr.edu
Thu Jul 26 14:48:40 UTC 2018
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 <ghc-steering-committee at haskell.org> 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 <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`
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>
>
> 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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