listing installed packages by source

[christophe barbe <christophe@cattlegrid.net>]

Is there an easy way to list all installed packages by source (main,
contrib, non-free, unknown ,,,)?
I am doing it with a dirty shell script using grep-status (to get the
list of installed package) and apt-cache to get the Filename field
value, but it's very slow. 
Is there a perl api for apt? (My understanding is that dpkg knows
nothing about the origine of the file)

Basically I want to know which non-free packages are installed on a
given computer but knowing which packages are not from the main archive
is interesting too.

Thanks,
Christophe

-- 
Christophe Barbé <christophe.barbe@ufies.org>
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