Smalltalk takes a hardline view on syntax errors and undefined variables: you can't save a new method until it's fixed.
It's a nice way to work, as methods tend to be small, so virtually all your code is in a runnable state all the time.
miniblog.
Related Posts
Declarative Debugging https://www.metalevel.at/prolog/debugging (Prolog is amazingly flexible in what you can comment out and still have a runnable program)
Trinket is a neat service for embedding runnable Python scripts on web pages https://trinket.io/ (largely targetting the edu community)
