Extending the Theory and Practice of Gradual Typing

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Programming languages are typically categorized as either statically or dynamically typed. Statically typed languages such as Java use types to ensure that a program satisfies basic behavioural guarantees before running it. Type checking catches programming errors and supports optimizations that make programs run faster and use less memory. However, programmers must add extra type annotations, and type checking often rejects programs that would otherwise run correctly. Dynamically typed languages such as Javascript and PHP use runtime checks to catch errors, so they enable flexible and fast-paced programming that is heavily used to develop web sites and mobile applications. But in trade they tend to run slower and use more memory and power, which is particularly bad for mobile apps, and deployed programs often harbour costly bugs that could have been detected right away by a type checker.