Dynamic typing in a statically-typed language

Statically-typed programming languages allow earlier error checking, better enforcement of disciplined programming styles, and generation of more efficient object code than languages where all type-consistency checks are performed at runtime. However, even in statically-type languages, there is often the need to deal with data whose type cannot be known at compile time. To handle such situations safely, we propose to add a type Dynamic whose values are pairs of a value v and a type tag T where v has the type denoted by T. Instances of Dynamic are built with an explicit tagging construct and inspected with a type-safe typecase construct. This paper is an exploration of the syntax, operational semantics, and denotational semantics of a simple language with the type Dynamic. We give examples of how dynamically-typed values might be used in programming. Then we discuss an operational semantics for our language and obtain a soundness theorem. We present two formulations of the denotational semantics of this language and relate them to the operational semantics. Finally, we consider the implications of polymorphism and some implementation issues.