bug#53518: 29.0.50; em-extpipe breaks input of sharp-quoted Lisp symbols

[Jim Porter]

On 1/25/2022 12:50 AM, Sean Whitton wrote:
> Hello again,
>
> On Mon 24 Jan 2022 at 06:39pm -08, Jim Porter wrote:
>
> > I just noticed a small bit of breakage with this. It's no longer
> > possible to refer to Lisp functions in Eshell like so:
> >
> >     #'upcase
> >
> > Eshell explicitly supports this construct (see `eshell-lisp-regexp'),
> > though it doesn't appear to be documented in the manual. Currently, this
> > syntax is only occasionally useful, but I'm working on a patch series
> > where it'll likely become a lot more common. My patches will add support
> > for piping to Lisp functions, so that you can do the following, for example:
> >
> >     $ echo hi | #'upcase
> >     HI
>
> Out of curiosity, why is there a need for the sharpquote?  Why not just
> 'echo hi | upcase'?  Is it to do with requesting the new piping?

It becomes more relevant with my WIP patches to support piping to Lisp 
functions, but it means something different today too. "upcase" *calls* 
the function `upcase', whereas "#'upcase" evaluates to the function 
object itself. For example:

  $ upcase
  Wrong number of arguments: #<subr upcase>, 0
  $ #'upcase
  upcase

Or, for a slightly different, but more practical example today:

  $ mapcar upcase $(list "foo" "bar")
  Invalid function: "upcase"
  $ mapcar #'upcase $(list "foo" "bar")
  ("FOO" "BAR")

In this case, "upcase" in the first command is just the string "upcase", 
whereas "#'upcase" refers to the function as in the other case. The 
shortest other way I'm aware of to spell that using shell-like 
invocation would be:

  $ mapcar $(quote upcase) $(list "foo" "bar")
  ("FOO" "BAR")

For testing purposes, it would probably also be useful to ensure that 
Lisp syntax works too:

  $ (mapcar #'upcase '("foo" "bar"))
  ("FOO" "BAR")

Just for the sake of completeness, in my WIP patches, "echo hi | 
#'upcase" and "echo hi | upcase" will also do different things, 
following the above precedent. The former pipes the output of echo to 
the function upcase. The latter pipes the output of echo to the *result* 
of calling the function upcase with no arguments. In addition to being 
consistent with how Eshell currently works, this allows you to do things 
like "echo hi | less -N", where "less -N" is evaluated as an Eshell 
command and then returns a pseudo-pipe for echo to connect to.