Mutable­Environments­Curtis­Cowan

This proposal provides mutable environments for R7RS-large. It's based on this proposal by Pavel Curtis, but he is not responsible for the use I have made of it.

Introduction

The small language provides four procedures that return global environment specifiers for use by eval. They are:

• environment, which returns an immutable environment specifier when passed on zero or more import-specs which contains the bindings made available by those import specs

• scheme-report-environment, which when passed the argument 5 returns a specifier for a copy of the R5RS environment, which may or may not be mutable

• null-environment, which when passed the argument 5 returns a specifier for a copy of the R5RS environment containing syntax keywords only, which may or may not be mutable

• interaction-environment, which returns a mutable environment specifier containing implementation-defined bindings, including at least those exported by the base library

Procedures

The following procedures allow an application to generate, examine, and mutate environment specifiers which can be used like those obtained from the R7RS-small procedures above.

(make-environment environment ...)

Returns a newly allocated mutable environment specifier that has imported into it the bindings of the environments.

(environment? obj)

Returns #t if obj is an environment specifier, and #f otherwise.

(mutable-environment? obj)

Returns #t if obj is a mutable environment specifier, and #f otherwise.

(environment-ref environment symbol)

If symbol is bound as a variable in the environment specified by environment, returns the value bound to it. If symbol is bound as a syntax keyword, returns an implementation-defined object. If symbol is not bound, returns #f.

(environment-bound? environment symbol)

Returns #t if symbol is bound in the environment specified by environment, and #f otherwise.

(environment-set! environment symbol value)

In the environment specified by environment, binds symbol as a variable whose value is value, discarding any existing binding. Returns an unspecified value.

(environment-for-each environment proc)

Invokes proc on each identifier bound in the environment specified by environment whose value comes from a call to environment-set!. Proc is passed the identifier and the value (or an unspecified value if the identifier is bound as a syntax keyword). Note that identifiers imported when the environment was created are not passed to proc unless their values have been changed. Returns an unspecified value.