source blocks DAG evaluation

[c4t0]

Hi,

Is it possible to have a dependency hierarchy of source blocks?

i.e.: in C, if you use libA from a compilation unit and that library needs 
libB, you don't need to include it in main.c

-> main.c
#include "libB.h"
-> libB.c
#include "libA.h"

you don't need to:
-> main.c
#include "libB.h"
#include "libA.h"

because library headers are closed under inclusion.

I haven't succeeded in doing the same in org-mode. Even after populating 
org-babel-library-of-babel.

Using #+call just doesn't work. Using :var is better, evaluates all, but there 
appears to lack session support, it doesn't check for cycles and it feels a 
little hacky

With #+call I need to do it like this:

#+name: libA
#+begin_src scheme :results none
(define hi "hello")
#+end_src

#+name: libB
#+begin_src scheme :results none
(define greetings (string-append hi ", " "to all the people!"))
#+end_src

here is my "main" I need to C-c C-c in each #+call line and write the :session 
that the code block uses in each one, and do it in the correct order. If I C-c 
C-c in libB first it won't eval because 'hi' is not defined.

#+call: libB[:session example]
#+call: libA[:session example]
#+begin_src scheme :session example :results output
(display greetings)
#+end_src

source blocks can be #+call(ed) but aren't closed under #+call (a source block 
can be called but then the callee won't) 

instead I would like to :

#+name: libA
#+begin_src scheme :results none
(define hi "hello")
#+end_src

#+call: libA
#+name: libB
#+begin_src scheme :results none
(define greetings (string-append hi ", " "to all the people!"))
#+end_src

#+call: libB
#+begin_src scheme :session example :results output
(display greetings)
#+end_src

- there shouldn't be needed to C-c C-c in the #+call line, evaluating the 
source block alone should suffice.
- there shouldn't be a need to write the :session
  - it should use the session of the user evaled block, unless specified 
otherwise

In the other hand, using :var with a dummy variable:

#+name: libA
#+begin_src scheme :results none
(define hi "hello")
#+end_src

#+name: libB
#+begin_src scheme :results none :var _=libA
(define greetings (string-append hi ", " "to all the people!"))
#+end_src

#+HEADER: :var _=libB
#+begin_src scheme :session example :results output 
(display greetings)
#+end_src

It evals libA then libB and finally the (display greetings) code.
But it fails, because the :session example is ignored. Even if I add a :session 
example to every source block (which would be really bad, sessión must be 
decided by the consumer) it doesn't work. I think that is because :var expects 
a value, so it just opens a new session to evaluate code every time.

Besides if there are any dependency cycles, it just fails with: Lisp nesting 
exceeds ‘max-lisp-eval-depth’

So if I'm right and there is not an implemented way to do this, how can we do 
it? Adding session support for :var? constructing a DAG of #+calls and then 
evaluating in order? maybe using a new header?

COD.