Manuel Pégourié-Gonnard wrote:
>> Remember, though, that *everything* xparse makes is \protected. So a
>> function created with xparse will be trying to do \peek_<whatever> in an
>> expansion situation.
> 
> AFAIK, the beginning of a cell in an alignment is a expansion situation and the
> \protected doesn't help here.

A quick test confirms this (I'm nothing like an expert on \halign!)

> Thinking of it, it's not only about peeking functions, but about the whole
> parser, which should (optionally) work purely by expansion, so that no
> unexpandable token appears before the development of the actual user macro, thus
> allowing to define and effectively use macros involving \omit or \noalign.
> 
> With current \newcommand, the "parser" has this property if there is no optional
> argument (for the simple reason that there is no parser in this case) and could
> have it if \@testopt were replaced with eg \FE@testopt from etextools in other
> cases.
> 
> My question is: is it doable/desirable to allow defining macros having this
> property with xparse? It may involve a lot of changes in the implementation,
> hence not be worth trying. However, I hope it was worth at least raising the
> question.

There is a bigger issue than simply \peek_<whatever> here. With
\newcommand, it is only the first argument that is ever checked, so it
is possible to do things in an expandable fashion. I assume that it is
feasible to do the same for the first n arguments. However, with xparse
there is the possibility to do:

\DeclareDocumentCommand \foo { o m o m } % Or whatever syntax!

which I don't see being easy (possible?) to do in an expandable way.
You've then got to factor in the parallel question about
post-processing: this would pretty certainly fail. (That is before you
factor in other argument types.)

I guess that using LuaTeX this type of thing becomes a lot easier.  I
wonder if this is a case where as Jonathan Fine suggests we are trying
to do things TeX macros simply cannot do because of the nature of TeX
(even with the various extensions).
-- 
Joseph Wright