Mathematica REPL

Put the following code into the Definition Window and then press the "Run" button:

(require (lib "mathematica.ss" "mrmathematica"))
(MathKernel)
(current-eval (lambda (x) (MathEval (syntax-object->datum x))))

Now the Interaction Window is working as a Mathematica REPL:

> (+ 1 2 3)
6
> (- 3 2 1)
0
> (D (sin x) x)
(cos x)
> (+ 2 x 1)
(+ 3 x)
> (Set x 5)
5
> (> x 0)
#t
> (if (> x 0) #f #t)
#f