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Source for wiki MutableEnvironmentsCurtisCowan version 5

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cowan

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127.11.51.1

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MutableEnvironmentsCurtisCowan

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This proposal provides mutable environments for R7RS-large. It's based on [http://groups.csail.mit.edu/mac/ftpdir/scheme-mail/HTML/rrrs-1988/msg00139.html 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.

== Constructor ==

`(make-environment `''environment'' ...`)`

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

== Predicates ==

`(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-bound? `''environment symbol''`)`

Returns `#t` if ''symbol'' is bound in the environment specified by ''environment'', and `#f` otherwise.

`(environment-syntax-keyword? `''environment symbol''`)`

Returns `#t` if ''symbol'' is bound as a syntax keyword in the environment specified by ''environment'', and `#f` otherwise.

`(environment-assigned? `''environment symbol''`)`

Returns `#t` if ''symbol'' is bound as a variable and is assigned a value in the environment specified by ''environment'', and `#f` otherwise.

== Accessors ==

`(environment-ref `''environment symbol''`)`

If ''symbol'' is bound as a variable that has been assigned a value 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 which specifies whatever is bound to it such that it can be passed to `environment-set!`. If ''symbol'' is not bound or not assigned, returns `#f`.

`(environment-imports `''environment''`)`

Returns a list of environments that have been imported into ''environment''. It is an error to mutate this list.

== Mutators ==

`(environment-bind `''mutable-environment symbol syntax-keyword?''`)`

Binds ''symbol'' in ''mutable-environment'' as a syntax keyword or variable. The value is unassigned.

`(environment-import! `''mutable-environment environment'' ...`)`

Modifies ''mutable-environment'' to import ''environments''. It is an error if any symbol is imported from more than one environment, either specified or already imported into ''mutable-environment''. Any symbols bound in ''mutable-environment'' shadow symbols in the imported environments.

`(environment-unimport! `''mutable-environment environment'' ...`)`

Modifies ''mutable-environment'' to not import ''environments''. Unimporting an environment that is not imported has no effect.

`(environment-set! `''mutable-environment symbol value''`)`

In ''mutable-environment '', assigns ''symbol'' (which must be bound as a variable or syntax keyword) to ''value''. It is an error if ''symbol'' is bound to a syntax keyword but ''value'' is not derived from a call on `environment-ref` passing an already existing syntax keyword. Returns an unspecified value.

`(environment-remove! `''mutable-environment symbol''`)`

In the mutable environment specified by ''environment'', removes any binding for ''symbol'' created by `environment-set!`, revealing any imported binding. If there is no such binding, it does nothing. Returns an unspecified value.

`(environment-freeze! `''environment''`)`

Causes ''environment'' to become an immutable environment.

== The whole environment ==

`(environment-for-each `''environment proc''`)`

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

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2015-12-07 05:55:16