A Brief Overview of the Pawns Programming Language

This paper describes the Pawns programming language, currently under development, which uses several novel features to combine the functional and imperative programming paradigms. It supports pure functional programming (including algebraic data types, higher-order programming and parametric polymorphism), where the representation of values need not be considered. It also supports lower-level C-like imperative programming with pointers and the destructive update of all fields of the structs used to represent the algebraic data types. All destructive update of variables is made obvious in Pawns code, via annotations on statements and in type signatures. Type signatures must also declare sharing between any arguments and result that may be updated. For example, if two arguments of a function are trees that share a subtree and the subtree is updated within the function, both variables must be annotated at that point in the code, and the sharing and update of both arguments must be declared in the type signature of the function. The compiler performs extensive sharing analysis to check that the declarations and annotations are correct. This analysis allows destructive update to be encapsulated: a function with no update annotations in its type signature is guaranteed to behave as a pure function, even though the value returned may have been constructed using destructive update within the function. Additionally, the sharing analysis helps support a constrained form of global variables that also allows destructive update to be encapsulated and safe update of variables with polymorphic types to be performed.