[Orgmode] What is output for org-babel?

[andrea Crotti]

I'm with org-mode 6.31a, I was trying to
get output in the exported files but I never see the output.

So I investigate and for example this

#+BEGIN_SRC python
import os; os.listdir(os.getcwd())
#+END_SRC

doesn't return any output while this

#+BEGIN_SRC python
"hello"
#+END_SRC

does and this:

#+BEGIN_SRC python
return "hello"
#+END_SRC
Again doesn't.

I guess that the returned value is depending also on the language,
but in general what's the rule?

Another little thing, I have a file with many haskell source blocks.
They are separated because they are different answers,
but if they are loaded separately they don't work.
So is there a way to take all the source code and load it at once?

Something like more or less:
(append-to-buffer "haskell-buf" (get-code "exercises.org"))