[ghc-steering-committee] Proposal #606: small syntax change around types in terms; recommendation: vote

[Moritz Angermann]

Maybe I'm misreading this, but this can potentially break existing code,
can it not?

On Thu, 25 Jan 2024 at 02:47, Richard Eisenberg <rae at richarde.dev> wrote:

> Proposal #606 <https://github.com/ghc-proposals/ghc-proposals/pull/606> has
> been submitted to the committee, by Vlad.
>
> This proposal makes a tiny change. Suppose we have `f :: forall (x ::
> Type) -> x -> ...` (this is part of -XRequiredTypeArguments). Then (as
> proposed) we can write `y = f (type Int -> Int) abs ...`. The `type`
> keyword here is to signal that what comes is a type: it should be parsed as
> a type and name-resolved as a type. This might matter if there is, say, a
> constructor Int in scope. The proposal at hand is whether to accept the
> above program or to require extra parentheses, thusly: `y = f (type (Int ->
> Int)) abs ...`. That's it -- that's the entire proposal. (It says we should
> _not_ accept the former, without parentheses.)
>
> The motivation is to avoid surprising users. Normally when we have `word1
> word2 -> word3`, word1 and word2 will associate more closely than the
> arrow. Yet the first example has `type Int -> Int` where the arrow binds
> more tightly. Because `type` is a keyword, this is no challenge to parse
> and is not ambiguous -- it's just perhaps confusing to users.
>
> There was some debate in the proposal, but in my perusal, not a clear
> indication toward any particular direction. The most rigorous statement I
> could find is that the new syntax is a subset of the original, and so we
> can easily reverse this decision later.
>
> I propose we vote on the matter. We have two choices:
>
> 1. Original syntax: allow `(type Int -> Int)` as an argument.
> 2. Amended syntax: require `(type (Int -> Int))` as an argument.
>
> --------------------------------
>
> My vote: 1.
>
> There are many keywords in Haskell, and we are used to parsing these
> differently. For example, if we have `f (do x <- blah ...)`, we know quite
> well that the <- is within the `do`, not the other way around. Ditto
> `case`: we don't require scrutinees to be parenthesized. I posit that the
> strangeness some have felt around `(type Int -> Int)` is (understandable)
> confusion in the face of novelty. But the `type` herald will not be novel
> forever, and I think we'll enjoy having fewer parentheses in our code in
> the long run. (I might be arguing for "2 today, then 1 tomorrow". But let's
> just skip the intermediate step and do 1 now.)
>
> I welcome your opinions and votes. It would be great to conclude this in
> the next 2 weeks, by Feb 7.
>
> Thanks!
> Richard
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