I posted this elsewhere in the thread, but local blocks that define which variables they read, mutate and export would IMO be a very good solution to this problem:
This is a fascinating idea. In some languages like C or Java or C#, the IDE can probably do this "for free" -- generate, then programmer can spot check for surprises. Or the reverse, highlight a block of code and ask the IDE to tell you about read/mutate/export. In some sense, when you use automatic refactoring tools (like IntelliJ), extract a few lines of code as a new method needs to perform similar static analysis.
In the latest IntelliJ, the IDE will visually hint about mutable, primitive-typed local variables (including method parameters). A good example is a for loop variable (i/j/k). The IDE makes it stand-out. When I write Java, I try to use final everywhere for primitive-typed local variables. (I borrowed this idea from functional programming styles.) The IDE gives me a hint if I accidentally forget to mark something as final.
It's similar, but lambdas don't specify the behaviour as precisely, and they're not as readable since the use of a lambda implies a different intention, and the syntax that transforms them into a scope block is very subtle. They may also have performance overhead depending on the environment, which is (arguably) additional information the programmer has to consider on usage.
