Ah, yes, quite right: since the projections match the whole patterns, the bang patterns in a constructor would be forced as soon as one of the fields in the constructor is used, so this also diverges:
ex3 = let (x, !y) = (5,undefined) in x The rule is consistent, to me it just seems quite unintuitive. On Thu, Sep 10, 2020 at 9:18 AM Richard Eisenberg <r...@richarde.dev> wrote: > This whole area is clearly somewhat troublesome: > > On Sep 10, 2020, at 12:05 PM, Iavor Diatchki <iavor.diatc...@gmail.com> > wrote: > > 3. nested bang patterns in pattern bindings should count as "uses" of the > value and therefore should be strict. For example if I write `let ( !x, !y > ) = undefined in ()`, I think this should be equivalent to `let (x,y) = > undefined in x `seq` y `seq` ()`. With the current behavior the bang > patterns don't do anything, while my guess would be that most people would > expect the suggested behavior instead. As usual, we should not allow that > at the top level. > > > This isn't quite right. > > Consider > > ex0 = let ( !x, !y ) = undefined in () > ex1 = let ( !x, !y ) = (5, undefined) in x > ex2 = let ( !x, y ) = (5, undefined) in x > > > ex0 converges, because let-bindings are lazy by default. > ex1 diverges, because the bang on y means that, when the patten-match > happens at all, x and y are bound strictly. So bangs *do* matter in nested > patterns within pattern bindings. By contrast, ex2 converges. > > Again, I'm not arguing in favor of the current behavior, but I want to > make sure we're all as informed as possible in this debate. > > Richard >
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