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 run time. However, even in statically typed languages, there is often the need to deal with data whose type cannot be determined 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 explores the syntax, operational semantics, and denotational semantics of a simple language including the type Dynamic. We give examples of how dynamically typed values can 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 s...

Compilers for monomorphic languages, such as C and Pascal, take advantage of types to determine data representations, alignment, calling conventions, and register selection. However, these languages lack important features including polymorphism, abstract datatypes, and garbage collection. In contrast, modern programming languages such as Standard ML (SML), provide all of these features, but existing implementations fail to take full advantage of types. The result is that performance of SML code is quite bad when compared to C. In this thesis, I provide a general framework, called type-directed compilation, that allows compiler writers to take advantage of types at all stages in compilation. In the framework, types are used not only to determine efficient representations and calling conventions, but also to prove the correctness of the compiler. A key property of typedirected compilation is that all but the lowest levels of the compiler use typed intermediate languages. An advantage of this approach is that it provides a means for automatically checking the integrity of the resulting code. An important