Well, there is a REPL, but it's not that great:

    $ nim i
    >>> for i in 0..10:
    ...   echo "Hello World"[0..i]
    ...
    H
    He
    Hel
    Hell
    Hello
    Hello 
    Hello W
    Hello Wo
    Hello Wor
    Hello Worl
    Hello World

To use up/down keys, build the Nim compiler with "./koch boot -d:release -d:useGnuReadline" or run "rlwrap nim i" instead.

Edit: It's even buggier than I remembered, best avoid it and use "nim -r c file" instead.