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

[Joachim Breitner]

Hi,

can we have an additional nod by someone who is not an author of the
corresponding paper?

Also, I updated the proposal to answer the disambiguation with as-
patterns:
https://github.com/ghc-proposals/ghc-proposals/pull/126/commits/12d64105b0979c674502c3768118b424d09a572b
And I would like a second pair of eyes there, as this choice blocks us
from in the future allowing patterns on both sides on @, i.e. something
like

   Just (_ :_) @ (fun -> True) @ x

because 

   Nothing @ a

would be ambiguous.

Cheers,
Joachim

Am Donnerstag, den 26.07.2018, 11:08 +0300 schrieb Iavor Diatchki:
> 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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-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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