F# hits a really nice sweet spot for this: scripts with an integrated interactive scripting environment, and complete access to a plethora of unit testing frame

F# hits a really nice sweet spot for this: scripts with an integrated interactive scripting environment, and complete access to a plethora of unit testing frameworks. That lets you work line-for-line through your code, inspect and analyze each step as needed, edit parameters as wanted, restart and do clean runs through calculations using custom and third party libraries, and also interactively execute any tests/specs you have defined. It's a very tight feedback loop that doesn't compromise any of the long term artifacts you might want in a serious app. In conjunction with .Nets extension mechanisms and some F# language features the setup lets you fire up a basic script, replace/extend anything from your object graph with new code, write accordant tests, and execute any or all parts of the test as desired in an interactive REPL-like session for problem solving and debugging: all in the same file :) This flows really nicely with the single pass compiler, keeping things nice and clean so they integrate into the main app once exploratory development is done. Semi-serious typing, functional composition, and dynamic exploration -- it ain't bad.