TIP #235: Exposing a C API for Ensembles

Title:Exposing a C API for Ensembles
Version:$Revision: 1.9 $
Author:Donal K. Fellows <donal dot k dot fellows at manchester dot ac dot uk>
Created:Friday, 10 December 2004


This TIP exposes a C API for the ensembles of TIP #112.


As the Tcl core moves to make more use of ensembles, it becomes more necessary for there to be an API available to allow control over those ensembles from the C level so that it is not necessary for such manipulation to be done through the evaluation of Tcl scripts (e.g. because this interferes with the interpreter's command count and that may be limited in that particular interpreter). Given such an API, it should be exposed to code outside the Tcl core if at all possible.

Proposed Change

This TIP proposes exposing the following functions:


Create an ensemble linked to a particular namespace and return the command token for the namespace command. The flags argument specifies the initial version of the set of flags controllable through Tcl_SetEnsembleFlags (see below).

Tcl_Command Tcl_CreateEnsemble(Tcl_Interp *interp, const char *name, Tcl_Namespace *namespacePtr, int flags)


Return a command token for an ensemble, or NULL if the name does not refer to an ensemble. The flags argument controls whether failure to find an ensemble is an error and should be either 0 or TCL_LEAVE_ERR_MSG.

Tcl_Command Tcl_FindEnsemble(Tcl_Interp *interp, Tcl_Obj *cmdNameObj, int flags)


Test whether a command token refers to an ensemble.

int Tcl_IsEnsemble(Tcl_Command command)

Tcl_GetEnsembleSubcommandList, Tcl_SetEnsembleSubcommandList

Read and write the (fixed) list of subcommands of the ensemble. Equivalent to manipulating the -subcommands option.

int Tcl_GetEnsembleSubcommandList(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj **subcmdListObjPtr)

int Tcl_SetEnsembleSubcommandList(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj *subcmdListObj)

Tcl_GetEnsembleMappingDict, Tcl_SetEnsembleMappingDict

Read and write the dictionary that maps subcommands to prefixes of implementation commands. Equivalent to manipulating the -map option.

int Tcl_GetEnsembleMappingDict(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj **mapDictObjPtr)

int Tcl_SetEnsembleMappingDict(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj *mapDictObj)

Tcl_GetEnsembleUnknownHandler, Tcl_SetEnsembleUnknownHandler

Read and write the command prefix list that is used to implement the unknown-subcommand handler. Equivalent to manipulating the -unknown option.

int Tcl_GetEnsembleUnknownHandler(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj **handlerListObjPtr)

int Tcl_SetEnsembleUnknownHandler(Tcl_Interp *interp, Tcl_Command command, Tcl_Obj *handlerListObj)

Tcl_GetEnsembleFlags, Tcl_SetEnsembleFlags

Read and write the flags for the ensemble. Currently there is only one settable flag bit, TCL_ENSEMBLE_PREFIX, which is present exactly when -prefix option is true.

int Tcl_GetEnsembleFlags(Tcl_Interp *interp, Tcl_Command command, int *flagsPtr)

int Tcl_SetEnsembleFlags(Tcl_Interp *interp, Tcl_Command command, int flags)


Reads the namespace associated with the ensemble; this value may not be altered.

int Tcl_GetEnsembleNamespace(Tcl_Interp *interp, Tcl_Command command, Tcl_Namespace **namespacePtrPtr)


The handle for an ensemble is a normal Tcl_Command handle, and any operation valid on such a command (e.g. deleting it or finding its name) is valid. Ensembles are just a special subclass of commands. This allows the ensemble API to be considerably sparser than it would otherwise have to be.

Reference Implementation

The API is already in the core, but with internal naming and not exposed to anyone. Exposure is just renaming a few functions and putting them in the stubs table. No new types are exposed. All this means that a reference implementation is truly trivial.


This document has been placed in the public domain.

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