let-in if do-expr is problematic? (was: Re: proposal: let in if parentheses)

[Isiah Meadows]

Just as a heads up, something semantically identical to this is what
prompted this big shutdown from Mark Miller, and is why the bar raised
pretty high since for new features:

https://esdiscuss.org/topic/the-tragedy-of-the-common-lisp-or-why-large-languages-explode-was-revive-let-blocks
On Tue, Aug 21, 2018 at 12:20 Herbert Vojčík <herby at mailbox.sk> wrote:

> Hi!
>
> It would be nice to know if do expressions have some a chance, otherwise
> some other syntax for let-in would be really helpful, especially now
> that we have arrow functions.
>
> I would propose to use different variant of let (maybe also const):
>
> OP 1:
>
>    let in a = b(), if (a) a.c();
>
> OP 2:
>
>    let in a = b(), if (a) c(a);
>
> Instead of
>    const big = raw => {
>      let cooked = cook(raw);
>      return consumer => {
>        // do things with consumer and cooked
>      };
>    };
>
>    const big = raw =>
>      let in cooked = cook(raw), consume => {
>        // do things with consumer and cooked
>      };
>
> In short,
>
>    let in binding = expr, stmt|expr
>
> It may work for `const in` as well.
>
> Herby
>
> P.S.: Alternative syntax is "let a=3, b=4, ..., in foo(a,b,c,d)" but
> this can only tell late if it is plain let-up-to-end-of-scope or
> local-scope-let, so not sure if that may be a problem; OTOH you can
> chain more of them and resembles classical let-in better.
>
> Isiah Meadows wrote on 21. 8. 2018 20:17:
> > It's possible, but the ability to optionally destructure is what would
> > make this feature worth it - I feel this should wait for pattern
> > matching to be added first, though.
> >
> > On Tue, Aug 21, 2018 at 11:10 Jordan Harband <ljharb at gmail.com
> > <mailto:ljharb at gmail.com>> wrote:
> >
> >     ```
> >     {
> >        let a = b();
> >        if (a) {
> >          c(a);
> >        }
> >     }
> >     ```
> >
> >     On Tue, Aug 21, 2018 at 10:54 AM, Ali Rahbari <rahbari at gmail.com
> >     <mailto:rahbari at gmail.com>> wrote:
> >
> >         while it's possible to use let keyword in for loop parentheses,
> >         it's not possible to use it in if parentheses.
> >
> >         There are two use cases for this:
> >
> >         *1- if (let a = b()) a.c();*
> >           this can be done using optional chaining which is proposed:
> >         b()?.c();
> >
> >         *2- if (let a = b()) c(a);*
> >         this or more sophisticated patterns can't be done in any way
> >         other than this:
> >         let a = b();
> >         if (a) c(a);
> >
> >         the problem here beside more line of codes, is *a *is defined
> >         outside of if scope.
> >
> >
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