Re: Is it really necessary to allow operators and whole right hand sides to be substituted in (())?

[Lawrence Velázquez]

On Mon, May 17, 2021, at 9:31 PM, Peng Yu wrote:
> I would like to know if this decision is based on any logic reasoning.

You could try asking David Korn.

> But it seems to be more based on convenience of implementation as when
> it is implemented in this way, the implementation for math can be
> partly shared with other parts.

Convenience of implementation is not nothing.

The current state of affairs also maintains consistency with
non-arithmetic contexts (single quotes and literal here-documents
aside).

> The reason that I want to understand the rationale is that things like
> these seem to make shell code inherently hard to optimize for speed,
> because there is no way to know what the expression expands to until
> the code actually runs. If speed were a concern for the math
> operations

It isn't.

> >> a=4
> >> b=a
> >> c=b
> >> ((d = c * 2))
> >> echo "$d"    # output: 8
> 
> Also, is there a real situation such multi-level reference is really
> useful.

What alternative would you prefer?  That ((d = c * 2)) resolve to
((d = b * 2)), treat b as 0, and set d to 0?  That it consider "b"
an error?  Or what?

> Just to be sure, `(())` and `let` are not part of POSIX but bash
> extensions of POSIX shell?

As Chet already said, they were introduced by ksh.

-- 
vq