[ghc-steering-committee] Record dot notation

[Simon Marlow]

On Fri, 7 Feb 2020 at 22:37, Joachim Breitner <mail at joachim-breitner.de>
wrote:

>
> I really would prefer a design where all these questions do not even
> need to be asked…
>

Me too. Also what about (.x) vs. ( .x), are those the same?


> So I think to have the full picture, we need the following option as
> well on the ballot:
>
>  5. .x is a postfix operator, binding exactly like application,
>     whether it is naked or not.
>     (This is option 3, but without the whitespace-sensitivity.)
>

[...]


> Anyways, now for my opinion: Assuming no more options are added, my
> ranking will be
>
>   5 > 4 > 2 > 1 > 3
>
> This puts first the two variants where .x behaves like an existing
> language feature (either like function application or like record
> updates), has no whitespace sensitivity, and follows existing languages
> precedence (JS and OCaml, resp.).
> Then the compromise solution that simply forbids putting spaces before
> .x (so at least the program doesn't change semantics silently).
> I dislike variant 3, which adds a _new_ special rule, and where adding
> a single space can change the meaning of the program, so I rank that
> last.
>

I'm also against whitespace-sensitivity and I lean towards this ordering
too.
But I'm going with:

5 > 2 > 1 > 4 > 3

Rationale: (5) seems the easiest to explain and has the fewest special
cases, yet covers the use-cases we're interested in. Beyond that I want to
be conservative because I find it hard to predict the ramifications of the
more-complex alternatives 4/3, so I've put 2/1 ahead of those. I've made my
peace with the current record selection syntax binding more tightly than
application, and indeed I often rely on it to avoid a $, so I'm OK with 4
over 3.

Cheers
Simon



>
>
> Cheers,
> Joachim
>
>
> PS, because its on my mind, and just for fun:
>
> Under variant 3, both foo1 and foo2 typecheck, they do quite different
> things (well, one loops).
>
>   data Stream a = Stream { val :: a, next :: Stream a }
>
>   foo1 f s = Stream (s.val) (foo1 (fmap f s).next)
>   foo2 f s = Stream (s.val) (foo2 (fmap f s) .next)
>
>
> --
> Joachim Breitner
>   mail at joachim-breitner.de
>   http://www.joachim-breitner.de/
>
>
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