Microflow Expression Console

I'd love to have an integrated playground where I can test arbitrary microflow expressions to make sure my syntax is right before testing in the live app.

For example, when I'm doing substring operations, I'd like to be able to enter the following code somewhere and see the result. During creation of the microflow would be helpful, and if I could do this during the execution (when paused on a breakpoint) so that I have access to the variables in scope at that point in the mciroflow would be incredible too!

maybe something like 

> substring('Toronto', find('Toronto', 'n'))
==> 'nto'