Re: [PATCH] Add :noweb-prefix and :noweb-trans babel header arguments

[Ihor Radchenko]

Sébastien Miquel <sebastien.miquel@posteo.eu> writes:

> The attached patch adds support for two new babel header arguments:
> =:noweb-prefix= and =:noweb-trans=.
>
> =:noweb-prefix= can be set to =no= to disable the noweb prefix
> behaviour, where prefix characters are repeated when expanding a
> multiline noweb reference.

Thanks for the patch! The idea about :noweb-prefix looks useful.

> =:noweb-trans= can be set to =prin1-to-string= to insert a lisp string
> representing the content of the referenced src block.
>
> The goal is to allow one to use, say, a LaTeX src block to represent
> some LaTeX snippet to be tangled into a string in some lisp (or other)
> code. This isn't possible currently, and one has to manually string
> escape the LaTeX code.
> ...
> I've left undocumented the possibility of setting =:noweb-trans= to
> another function. I wonder if anyone can think of some other use.

prin1-to-string is too specific and only solves a single use-case.

Consider the following example:

#+BEGIN_SRC emacs-lisp :noweb yes :tangle yes :noweb-prefix no :noweb-trans 
prin1-to-string
<<preamble>>
(setq latex-header <<nw>>)
#+END_SRC

There are two noweb references here. Setting source block-wide
:noweb-trans is not helpful because the first reference will be
incorrectly filtered through prin1-to-string.

I'd rather introduce a new syntax to transform the noweb reference
inline. Something like

#+BEGIN_SRC emacs-lisp :noweb yes :tangle yes :noweb-prefix no
<<preamble>>
(setq latex-header <<(prin1-to-string nw)>>)
#+END_SRC

>  Noweb insertions honor prefix characters that appear before the noweb
> -syntax reference.  This behavior is illustrated in the following
> -example.  Because the =<<example>>= noweb reference appears behind the
> -SQL comment syntax, each line of the expanded noweb reference is
> -commented.  With:
> +syntax reference. This behavior can be turned off by setting the
> +=noweb-prefix= header argument to =no= and is illustrated in the
> +following example. Because the =<<example>>= noweb reference appears
> +behind the SQL comment syntax, each line of the expanded noweb
> +reference is commented. With:

This sounds a bit confusing. I would also add an example where it is
useful to set :noweb-prefix to no.
  
Best,
Ihor