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

I hate to throw a wrinkle in, but much of this discussion assumes that type arguments come before other arguments after a constructor. This is true today, and there are no immediate plans to change it, but this assumption won't be valid if we have full dependent types. For example, we might want something like

data T where MkT :: forall k -> Int -> forall (a :: k). TypeRep a -> T

Now, we might get patterns like

foo (MkT k n @a r) = ...

Without whitespace-sensitivity, I don't think we'll be able to disambiguate between type pattern and as-pattern. (I thought for a moment that we could use the constructor arity as a guide, but this fails if we have two @-signs, where only one is meant to be a type pattern. We won't be able to tell which one!) Whitespace-sensitivity is bad, but it's not unprecedented in Haskell. Ignoring the layout rule (which is quite different), this is much more like the treatment of `.` and `$`. Specifically, `T . f` is T composed with f, while `T.f` is the name `f` from the module `T`. And `f $ (g x)` is f applied to (g x) while `f $(g x)` is f applied to a splice. Considering this future of relaxed ordering requirements on data constructors isn't compulsory, but I do think it's better if we don't paint ourselves into a corner around this. Thanks, Richard

On Aug 16, 2018, at 7:27 PM, Joachim Breitner wrote:

Hi,

Am Donnerstag, den 16.08.2018, 23:22 +0000 schrieb Simon Peyton Jones:

...

...

pattern Ascii :: String pattern Ascii <- (all isAscii -> True)

and now

foo (Ascii @ ('X':rest)) =

is now longer as silly…

That’s a good example. It's easy to work around:

pattern And p q = p@q

foo (Ascii `And` ('X' : rest)) = ..

nice. I included this in the proposal under “Costs and Drawbacks”, for future reference.

Cheers, Joachim

-- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/ _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee