Scheming environments

by Edward Z. Yang

Environments are first-class objects in MIT/GNU Scheme. This is neat, because an integral part of the lexical structure of a Scheme is also a data-structure in its own right, able to encode data and behavior. In fact, the environment data structure is precisely what Yegge calls property lists, maps that can be linked up with inheritance. So not only is it a data structure, it's a highly general one too.

Even without the ability to pass around an environment as a first class variable, you can still leverage its syntactic ties to stash away local state in an object-oriented manner. The data is only accessible inside procedures that pointed to the appropriate environment frame, and traditionally the closure returns a lambda (with its double-bubble pointed to the newly born environment frame) that acts as the only interface into the enclosing state. This requires a fair amount of boilerplate, since this lambda has to support every possible operation you might want to do to the innards of the function.

With a first class environment object, however, you can futz around with the closure's bindings arbitrarily. Unfortunately, there's no way to directly get-current-environent (except for the top-level REPL environment, which doesn't count), so we resort to the following trick:

(procedure-environment (lambda () '()))

procedure-environment can grab the environment pointer from the double-bubble for some procedure. So, we force the creation of a double-bubble pointing to the environment we care about with the empty lambda.

I most recently used this technique in a 6.945 project, in which I used a lambda to generate a bunch of procedures with various parameters swapped out (encouraging code-reuse), something akin to the time-honored trick of including C files multiple times with different macro definitions. Instead of returning these procedures as a hash-table which then people would have to explicitly call, I just returned the environment, and thus any consumer could enter "a different universe" by using an appropriate environment.

Scheme is pretty unique in its celebration of environments as first-class objects. I could try this trick in Python, but the func_closure attribute on functions is read-only, and also Python has some pretty lame scoping rules. A shame, since this technique allows for some lovely syntactic simplifications.

Comment. Oleg Kiselyov writes in to mention that "MIT Scheme specifically is unique in its celebration of environments as first class environments," and notes that even some developers of MIT scheme have second thoughts about the feature. It makes code difficult to optimize, and is both theoretically and practically dangerous: theoretically dangerous since environments are really an implementation detail, and practically dangerous because it makes it very difficult to reason about code.

From in-person discussions, I'm not surprised that Sussman's favored dialect of Scheme allows such a dangerous feature; Sussman has always been in favor of letting people have access to dangerous toys and trusting them to use them correctly.