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

I do not feel strongly. I am content for Arnaud to make a choice, and make sure that it is clearly expressed in the final version of the spec. One small correction to Simon's description: this is all about non-variable

patterns. A variable pattern is allowed to be linear (e.g. let %1 x = f y in ...) even without a bang or other source of strictness.

I did not know that. Can the proposal spell this out explicitly please? Do newtypes make a difference? E.g let N x = e in ... where N is the data contructor of a newtype? Simon On Wed, 24 Jan 2024 at 14:07, Richard Eisenberg wrote:

I find Simon's rules put too much emphasis on syntax (the appearance of a ! mark), whereas I'm arguing in favor of basing the rules on semantics (is the binding strict). But I'm happy to concede the point; I just think my proposal is actually simpler (and should be so in the implementation, but I might be wrong).

One small correction to Simon's description: this is all about non-variable patterns. A variable pattern is allowed to be linear (e.g. let %1 x = f y in ...) even without a bang or other source of strictness.

Richard

On Jan 22, 2024, at 4:29 PM, Simon Peyton Jones < simon.peytonjones@gmail.com> wrote:

I suggest:

- A pattern pat is called *banged *iff it is of form !pat, or (pat) where pat is banged. - A pattern binding pat = rhs, *without a user-specified multiplicity annotation* - has multiplicity Many ifpat is not banged - If p is banged, it gets an inferred multiplicity (somehow). - In a pattern binding %p pat = rhs, *with an explicit user-specified multiplicity annotation %p* - the pattern pat must be banged regardless of p.

I don't think patterns in case branches are involved. These rules concern pattern bindings pat=rhe

Simon

On Mon, 22 Jan 2024 at 10:00, Arnaud Spiwack wrote:

...

On Mon, 22 Jan 2024 at 10:04, Simon Peyton Jones < simon.peytonjones@gmail.com> wrote:

...

Richard seems to be the only one with a strong opinion on this. I'm happy to implement Richard's recommendations (both in the proposal and in the code), unless there are dissenting voices?

I am still struggling to understand the choice that you are putting before us. I laid out my understanding last week:

- I think that the issue you are debating only arises when you, the programmer, want to *explicitly annotate *a let/where binding with a multiplicity. Right? - I think that you are saying that all bindings with a *non-Many *multiplicity *must *be strict. - I think the choices you are identifying are: - If the explicit annotation is Many, does the binding still need to be strict? - If the binding needs to be strict, do we insist on an explicit ! or, if -XStrict is on can we omit that bang?

Can I ask if my understanding is correct?

It is, with two caveats: - the question of whether a pattern is strict also shows up when we infer the multiplicity of an unannotated let-binding. In this case, if the non-variable pattern isn't strict, then we emit a multiplicity-must-be-Many constraint. And the same choice (should we consider some patterns as strict when they aren't annotated with `!`) arises. - beside the -XStrict question, there is the question of whether `(!p)` (with parentheses) is considered strict.

Here's the full logic I've considered so far (where `checkManyPattern` means: must be Many, and `return WpHole` means: no constraint on the multiplicity):

manyIfLazy dflags lpat | xopt LangExt.Strict dflags = xstrict lpat | otherwise = not_xstrict lpat where xstrict (L _ (LazyPat _ _)) = checkManyPattern pat_ty xstrict (L _ (ParPat _ _ p _)) = xstrict p xstrict _ = return WpHole

not_xstrict (L _ (BangPat _ _)) = return WpHole not_xstrict (L _ (VarPat _ _)) = return WpHole not_xstrict (L _ (ParPat _ _ p _)) = not_xstrict p not_xstrict _ = checkManyPattern pat_ty

...

Assuming so, to me the most straightforward and conservative thing is:

- All multiplicity-annotated pattern bindings must have an explicit bang, thus let %p !pat = rhs

This rule is simple, direct, and explicable. No one is going to write a multiplicity-annotated pattern binding let %Many pat = rhs, so it's not worth optimising for that case.

It's certainly the more conservative option. But Richard was arguing that it's too conservative. As for explicability: which of “the pattern must be strict” or “the pattern must be annotated with a !” is easier to explain? I'm not sure, but here's some food for thought: if we settle on “annotated with a !”, then how do we explain that patterns in case branches don't need to be annotated with a “!"?