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

[Simon Peyton Jones]

I support acceptance.  Let's land this soon.

Simon

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

> Hi all,
>
> I've reviewed Arnaud's
> 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
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