Re: [ghc-steering-committee] Proposal #624 on linear let-bindings; recommendation: accept

What's simplest is not obvious. - Only accepting concrete `!pat` if there's a %p gives simpler error messages, but is a few lines of code extra (and you may end up with lets that are inferred linear but you can't add `let %1` on them if the inference algorithm uses a more clever definition of strict) - Considering a pattern as strict when `-XStrict` or in brackets, is a few extra lines of code but maybe more intuitive. In any case, I consider all these choices to be quite comparable in simplicity from a code perspective (and for the record, the least amount of work for me is to use both alternatives, because they're the version that I've already implemented). What does the user consider simplest? On Sat, 13 Jan 2024 at 21:00, Adam Gundry wrote:

I haven't looked at this closely, but I'm happy to trust Arnaud and Richard's judgment and accept, especially since LinearTypes is at a very experimental stage.

Adam

On 12/01/2024 10:08, Arnaud Spiwack wrote:

...

At Richard's prompting, I've added the following alternative to the proposal (the current proposal is the most conservative of the two, which we can choose to stick to if we're unsure). I'm copying the alternative here because rendering seems to be broken on Github right now.

I'm rather agnostic on which side we choose, to be honest. Anyone with medium-to-strong opinions on the question?

Restrictions of multiplicity-annotated let bindings ---------------------------------------------------

The proposal specifies that a multiplicity annotated non-variable let binding ``let %p pat`` must be such that ``pat = !pat'`` even if ``p = 'Many``. It is easy to lift this restriction on two dimension:

- We can say, instead, that patterns not of the form ``!pat'`` emit a ``p ~ 'Many`` constraint instead. They already do (for the sake of inference), so this is strictly less code. - We can generalise to more strict patterns. For instance, we don't need to require a ``!`` if ``-XStrict`` is on, we can have patterns of the form ``(!pat')`` (with additional parentheses). This is a few lines of codes, inference actually already does this in my implementation, so it's already paid for (though it does annoyingly mostly duplicate another definition of strict pattern which I couldn't find a way to factor as a single function, I don't like this).

The reason that motivated the stronger restriction is to improve error messages, because we can then error out with “multiplicity-annotated let-bound patterns must be of the form !pat”, instead of the more mysterious “Couldn't unify 'Many with 'One” (see `#23586 <https://gitlab.haskell.org/ghc/ghc/-/issues/23586 https://gitlab.haskell.org/ghc/ghc/-/issues/23586>`_). But maybe the additional restrictions are more surprising than the error messages are helpful.

On Mon, 8 Jan 2024 at 10:07, Simon Peyton Jones mailto:simon.peytonjones@gmail.com> wrote:

I support acceptance. Let's land this soon.

Simon

On Sat, 6 Jan 2024 at 04:45, Richard Eisenberg mailto:rae@richarde.dev> wrote:

Hi all,

I've reviewed Arnaud's https://github.com/ghc-proposals/ghc-proposals/pull/624 https://github.com/ghc-proposals/ghc-proposals/pull/624 and wish to recommend acceptance.

The proposal is an amendment to the proposal for linear types, adding support for linear let bindings.

Today, if you have

f :: T %1-> T f t = let t2 = t in t2

you'll get an error because t2 is not linear. The only way to bind a linear variable is via a `case`, never a `let` or `where`. This is annoying. With this proposal, the little program above is accepted, with an inferred linearity restriction on t2. Users can also annotated their lets like `let %1 x = ... in ...`. Bindings in `where` clauses can also be inferred or annotated as linear.

There is a downside, of course: linear bindings have various restrictions, chiefly that they must be strict bindings (because projections are hard with linear types) and that bindings cannot be generalized. I'm a little unsure that the choices in the proposal (particularly around generalization) are the best for users, but I think the best way to learn is to experiment. In my understanding, the community knows that -XLinearTypes is subject to revision, and so I think we should just blast ahead, revising if and when necessary.

Please share your thoughts!

Thanks, Richard

-- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/

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