The ecology of LISP or the case for the preservation of the environment

Abstract

A new binding model for dynamically scoped functional languages (such as LISP) is presented. The scheme attempts to combine the fast variable access of shallow binding with the functional advantages of deep binding without placing a high cost on context-switching. Tags are used to label the nodes of the environment tree, and this is used to determine whether a given node is an ancestor of the current node, and thus which value of a variable is valid in the current environment. The efficiency of the new method is compared with the classical models (shallow and deep), and then viewed in the light of more recent developments such as the cache-cell system of MDL and Baker's rerooting scheme.

Detalls of the current implementation are described briefly, followed by a discussion of the implications of the model for garbage collection — in particular the need to regenerate tags on the environment tree — and how the scheme will affect the compilation process.