I've never gone further than thinking about it, but I've always been curious as to how simple it would be to use Awk as an interpreter for a really simple Tcl-like language:

    set a 1
    set b 2

    define add (n,m) $n + $m

    set result [add a b]

I think it would be simple enough to come up with some Awk pattern/actions to parse the above and execute the commands.