Union types allow to capture the possibility of a term to be of several possibly unrelated types. Traditional static approaches to union types are untagged and tagged unions, which present dual advantages in their use. Inspired by recent work on using abstract interpretation to understand gradual typing, we present a novel design for union types, called gradual union types. Gradual union types ...

Refinement type systems have been proposed by a number of researchers to sharpen the guarantees of existing type systems. Examples are systems that distinguish empty and non-empty lists by type, taint tracking and information flow control, dimension analysis, and many others. In each case, the type language is extended with annotations that either abstract semantic properties of values beyond t...

Dylan is an object-oriented language with a combination of features that present both difficulties and opportunities for efficient compilation. The language is thoroughly objectoriented and supports multimethod dispatch as well as dynamic typing. The semantics of the language are consistently defined in terms of a program runtime model. However, the design goals included providing significant o...