How should empty aliases work?

How should empty aliases work?

[Bart Schaefer]

Documentation says that if an alias ends in a space, then the next
word is also subject to aliasing.

Consider:

% alias empty=''
% alias output='empty echo'
% alias echo='print -r bar'
% output foo

What should happen?  The doc would lead one to expect:
1. "output" expands to "empty echo"
2. "empty" expands to nothing
3. nothing does not end in a space, so the following word ("echo") is
not expanded
4. "echo foo" is executed

What actually happens is that "echo" expands and "print -r bar foo" is
executed.  If that's correct, can someone explain why?

Re: How should empty aliases work?

[Bart Schaefer]

On Fri, Dec 29, 2017 at 12:30 AM, Bart Schaefer
<schaefer@brasslantern.com> wrote:
>
> % alias empty=''
> % alias output='empty echo'
> % alias echo='print -r bar'
> % output foo
>
> What actually happens is that "echo" expands and "print -r bar foo" is
> executed.  If that's correct, can someone explain why?

I think the answer is that being a word in command position has
precedence over being a word preceded by an alias replacement.  Words
in command position are always alias-replaceable; words not in command
position are alias-replaceable if preceded by an alias replacement
that ended with a space.

So when "empty" disappears, it leaves "echo" in command position and
the first clause applies even though the second clause does not.

Re: How should empty aliases work?

[Peter Stephenson]

On Fri, 29 Dec 2017 17:14:38 -0800
Bart Schaefer <schaefer@brasslantern.com> wrote:
> On Fri, Dec 29, 2017 at 12:30 AM, Bart Schaefer
> <schaefer@brasslantern.com> wrote:
> >
> > % alias empty=''
> > % alias output='empty echo'
> > % alias echo='print -r bar'
> > % output foo
> >
> > What actually happens is that "echo" expands and "print -r bar foo" is
> > executed.  If that's correct, can someone explain why?
> 
> I think the answer is that being a word in command position has
> precedence over being a word preceded by an alias replacement.  Words
> in command position are always alias-replaceable; words not in command
> position are alias-replaceable if preceded by an alias replacement
> that ended with a space.
> 
> So when "empty" disappears, it leaves "echo" in command position and
> the first clause applies even though the second clause does not.

Yes, when we get to "echo" we have no remaining knowledge that we've
expanded an alias.  The documentation simply refers to an alias being
expanded "if it is in command position [or a global alias]", with no
reference to preceding aliases.  So I think everything is as it should
be.

Consider also

alias foo="echo alias expanded;"

% foo foo
alias expanded
alias expanded

which is just following the same rules.

pws