GridWorld

May someone give me a good overview of GridWorld, I seem to not understand certain aspects of it?

Myth: Type-checking is a substitute for testing.
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Again, absolutely not true. Although some people may use type-checking (static in particular) for testing program correctness, this is neither its intended purpose nor its limit of use. Type-checking is more about ensuring meaningful, well-defined execution than ensuring correctness. Even though you may hear strong static-typing advocates say, “When your program type-checks, you’ll often find that it just works”, this is simply not true for large, intricate programs. Although type-checking may help you find errors, it is not the same as testing. Thus, it is not a suitable substitute for testing.