That's a stupid name... REPL has had a precise meaning (Read-Eval-Print-Loop) for the last 50 years, and I don't know how one can choose to give that name to a project.
It's a REPL in the sense that when you click Run, the value of the current evaluation is used (Looped) as the input for the next expression, but this stretches the common understanding, and naming it this way fails to communicate.
The full page transition when you click Run loses the implicit history from a teletype or terminal scrollback. A shell works because you can see two or more values were visible at the same time. The second pane could then show the code of the currently selected value. You could even show the diff/patch between the values as well.
The 'Run' text should look like a button rather than a label, and be moved to the right. It might work even better to just evaluate the code on a timeout without an explicit gesture.
Edited: for more positive feedback.
some passionate debate here: