Chapter 9

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 Each thread has its own initial parameterization, which is extended functionally and superseded by parameterizations that are attached to a particular continuation mark.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

• MZCONFIG_ENV -- current-namespace (use scheme_get_env)

• MZCONFIG_INPUT_PORT -- current-input-port

• MZCONFIG_OUTPUT_PORT -- current-output-port

• MZCONFIG_ERROR_PORT -- current-error-port

• MZCONFIG_ERROR_DISPLAY_HANDLER -- error-display-handler

• MZCONFIG_ERROR_PRINT_VALUE_HANDLER -- error-value->string-handler

• MZCONFIG_EXIT_HANDLER -- exit-handler

• MZCONFIG_EXN_HANDLER -- current-exception-handler

• MZCONFIG_DEBUG_INFO_HANDLER -- debug-info-handler

• MZCONFIG_EVAL_HANDLER -- current-eval

• MZCONFIG_LOAD_HANDLER -- current-load

• MZCONFIG_PRINT_HANDLER -- current-print

• MZCONFIG_PROMPT_READ_HANDLER -- current-prompt-read

• MZCONFIG_CAN_READ_GRAPH -- read-accept-graph

• MZCONFIG_CAN_READ_COMPILED -- read-accept-compiled

• MZCONFIG_CAN_READ_BOX -- read-accept-box

• MZCONFIG_CAN_READ_TYPE_SYMBOL -- read-accept-type-symbol

• MZCONFIG_CAN_READ_PIPE_QUOTE -- read-accept-bar-quote

• MZCONFIG_PRINT_GRAPH -- print-graph

• MZCONFIG_PRINT_STRUCT -- print-struct

• MZCONFIG_PRINT_BOX -- print-box

• MZCONFIG_CASE_SENS -- read-case-sensitive

• MZCONFIG_SQUARE_BRACKETS_ARE_PARENS -- read-square-brackets-as-parens

• MZCONFIG_CURLY_BRACES_ARE_PARENS -- read-curly-braces-as-parens

• MZCONFIG_ERROR_PRINT_WIDTH -- error-print-width

• MZCONFIG_CONFIG_BRANCH_HANDLER -- parameterization-branch-handler

• MZCONFIG_ALLOW_SET_UNDEFINED -- allow-compile-set!-undefined

• MZCONFIG_CUSTODIAN -- current-custodian

• MZCONFIG_USE_COMPILED_KIND -- use-compiled-file-kinds

• MZCONFIG_LOAD_DIRECTORY -- current-load-relative-directory

• MZCONFIG_COLLECTION_PATHS -- current-library-collection-paths

• MZCONFIG_PORT_PRINT_HANDLER -- global-port-print-handler

• MZCONFIG_LOAD_EXTENSION_HANDLER -- current-load-extension

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:

• If arity is non-negative, potential parameter values must be able to accept the specified number of arguments. The check and expected arguments should be NULL.

• If check is not NULL, it is called to check a potential parameter value. The arguments passed to check are always 1 and an array that contains the potential parameter value. If isbool is 0 and check returns scheme_false, then a type error is reported using name and expected. If isbool is 1, then a type error is reported only when check returns NULL and any non-NULL return value is used as the actual value to be stored for the parameter.

• Otherwise, isbool should be 1. A potential procedure argument is then treated as a Boolean value.

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