[plt-scheme] A graphic toplevel

Hello,

I'm writting an lexer/parser with a "toplevel" read-eval-loop. I can replace
the toplevel scheme by this, and now i try to make it in graphic mode. I
find an idea with two editor-canvas and a button, i'm explain, in the first
editor the user write the question, and when i want to eval this question
he's click on the button and the result was written in the second editor.

But i want to automatics this system like the toplevel of Scheme, when the
user press enter the question was eval and written, but i can't find an mred
objets who can do this.

Can you help me ?

Thanks for avancy for your answer.