Parameterizations

A parameterization is a set of parameter values. Each thread has its own initial parameterization, which is extended functionally and superseded by parameterizations that are attached to a particular continuation mark.

Parameterization information is stored in a Scheme_Config record. For the currently executing thread, scheme_current_config returns the current parameterization.

To obtain parameter values, a Scheme_Config is combined with the current threads Scheme_Thread_Cell_Table, as stored in the thread record's cell_values field.

Parameter values for built-in parameters are obtained and modified (for the current thread) using scheme_get_param and scheme_set_param. Each parameter is stored as a Scheme_Object * value, and the built-in parameters are accessed through the following indices:

To get or set a parameter value for a thread other than the current one, use scheme_get_thread_param and scheme_set_thread_param, each of which takes a Scheme_Thread_Cell_Table to use in resolving or setting a parameter value.

When installing a new parameter with scheme_set_param, no check is performed on the supplied value to ensure that it is a legal value for the parameter; this is the responsibility of the caller of scheme_set_param. Note that Boolean parameters should only be set to the values #t and #f.

New primitive parameter indices are created with scheme_new_param and implemented with scheme_make_parameter and scheme_param_config.

9.1  Library Functions

¤ Scheme_Object *scheme_get_param(Scheme_Config *config, int param_id)

Gets the current value (for the current thread) of the parameter specified by param_id.

¤ Scheme_Object *scheme_set_param(Scheme_Config *config, int param_id, Scheme_Object *v)

Sets the current value (for the current thread) of the parameter specified by param_id.

¤ Scheme_Object *scheme_get_thread_param(Scheme_Config *config,
    Scheme_Thread_Cell_Table *cells,
    int param_id)

Like scheme_get_param, but using an arbitrary thread's cell-value table.

¤ Scheme_Object *scheme_set_thread_param(Scheme_Config *config,
    Scheme_Thread_Cell_Table *cells,
    int param_id, Scheme_Object *v)

Like scheme_set_param, but using an arbitrary thread's cell-value table.

¤ Scheme_Object *scheme_extend_config(Scheme_Config *base, int param_id, Scheme_Object *v)

Creates and returns a parameterization that extends base with a new value v (in all threads) for the parameter param_id. Use scheme_install_config to make this configuration active in the current thread.

¤ void scheme_install_config(Scheme_Config *config)

Adjusts the current thread's continuation marks to make config the current parameterization. Typically, this function is called after scheme_push_continuation_frame to establish a new continuation frame, and then scheme_pop_continuation_frame is called later to remove the frame (and thus the parameterization).

¤ Scheme_Thread_Cell_Table *scheme_inherit_cells(Scheme_Thread_Cell_Table *cells)

Creates a new thread-cell-value table, copying values for preserved thread cells from cells.

¤ int scheme_new_param()

Allocates a new primitive parameter index. This function must be called before scheme_basic_env, so it is only available to embedding applications (i.e., not extensions).

¤ Scheme_Object *scheme_register_parameter(Scheme_Prim *function, char *name, int exnid)

Use this function instead of the other primitive-constructing functions, like scheme_make_prim, to create a primitive parameter procedure. See also scheme_param_config, below. This function is only available to embedding applications (i.e., not extensions).

¤ Scheme_Object *scheme_param_config(char *name, Scheme_Object *param,
    int argc, Scheme_Object **argv,
   int arity, Scheme_Prim *check, char *expected,
   int isbool)

Call this procedure in a primitive parameter procedure to implement the work of getting or setting the parameter. The name argument should be the parameter procedure name; it is used to report errors. The param argument is a fixnum corresponding to the primitive parameter index returned by scheme_new_param. The argc and argv arguments should be the un-touched and un-tested arguments that were passed to the primitive parameter. Argument-checking is performed within scheme_param_config using arity, check, expected, and isbool:

This function is only available to embedding applications (i.e., not extensions).