Something that would be cool is a purely functional programming language with an interpreter and editor that can give feedback on the execution of the code as r

Something that would be cool is a purely functional programming language with an interpreter and editor that can give feedback on the execution of the code as rewrites. If "purely" starts to ring alarm bells for you, I guess you could let this interactive editing be 'purely functional', with the output of the editor as side effects, while having a separate mode of execution that is compiled (you might want to actually compile the code instead of running it interpreted, anyway) which is truly pure.