Static types

There are two possible times when type errors can happen: at compile time and at runtime. This is the crucial difference between static and dynamic type systems: static type systems are so called because they statically analyze programs and try to find type errors, while dynamic type systems are so called because they throw type errors dynamically, while they run the program.

A static type system will try to find as many errors as possible without running your program. Normally, this is not a guarantee that you will catch all type errors before runtime. Some errors may slip through the typechecker and still hit you at runtime. Plus, a type system may make it difficult to express a program that you know is correct because it thinks it's not. You will want to pay close attention when this happens, though, as either the typechecker is correct, or your design will benefit from expressing the program in a different, accepted, way.