refinement types

This is a further development of the idea of tag types.

There are situations where being able to encode a certain predicate about a given value using its type is tremendously helpful.

Let's imagine that we have a number. In order to proceed with it we need to assert that it is greater than 0. This assertion doesn't come for free, because it takes some processor time to get evaluated. Since numbers are immutable this assertion only needs to be evaluated once and will hold true from then on without having to be reevaluated. But due to having no way to attach this knowledge to the number value we:

• either need to play safe and reevaluate it every time immediately before using
• or take our chances trying to trace it through the code from the place where it was asserted keeping the result of the assertion in mind and hoping not to break things during the next refactoring

Being able to attach additional knowledge to the type of a value in form of predicate that is guaranteed to hold true as long as the value doesn't change is what is called refinement types.

@ahejlsberg, what are the chances of seeing the refinement types in TypeScript one day?

References:

• Refinement types: https://en.wikipedia.org/wiki/Refinement_(computing)
• Pre/post conditions, Hoare logic: https://en.wikipedia.org/wiki/Hoare_logic
• Design by contract: https://en.wikipedia.org/wiki/Design_by_contract

Start by reading the issue's tag types link and its references on refinement types, Hoare logic, and design by contract. The issue names no files, tests, or implementation entry point; a completed contribution would first need a defined TypeScript design and accepted scope for refinement types.