We motivate and discuss a novel functional programming construct
that allows convenient modular run-time nonstandard interpretation
via reflection on closure environments. This map-closure construct
encompasses both the ability to examine the contents of a
closure environment and to construct a new closure with a modified
environment. From the user’s perspective, map-closure is a
powerful and useful construct that supports such tasks as tracing,
security logging, sandboxing, error checking, profiling, code instrumentation
and metering, run-time code patching, and resource
monitoring. From the implementor’s perspective, map-closure
is analogous to call/cc. Just as call/cc is a non-referentiallytransparent
mechanism that reifies the continuations that are only
implicit in programs written in direct style, map-closure is a nonreferentially-
transparent mechanism that reifies the closure environments
that are only implicit in higher-order programs. Just as
CPS conversion is a non-local but purely syntactic transformation
that can eliminate references to call/cc, closure conversion
is a non-local but purely syntactic transformation that can eliminate
references to map-closure. We show how the combination
of map-closure and call/cc can be used to implement set! as
a procedure definition and a local macro transformation