Tool Command Language: 4.4 Syntax and Structure: Command Results

Each command produces two results: a code and a string . The code indicates whether the command completed successfully or not, and the string gives additional information. The valid codes are defined as follows:

TCL_OK: This is the normal return code, and indicates that the command completed successfully. The string gives the commands's return value.
TCL_ERROR: Indicates that an error occurred; the string gives a message describing the error.
TCL_RETURN: Indicates that the return command has been invoked, and that the current procedure should return immediately. The string gives the return value that procedure should return.
TCL_BREAK: Indicates that the break command has been invoked, so the innermost loop should abort immediately. The string should always be empty.
TCL_CONTINUE: Indicates that the continue command has been invoked, so the innermost loop should go on to the next iteration. The string should always be empty.

Tcl programmers do not normally need to think about return codes, since TCL_OK is almost always returned. If anything else is returned by a command, then the Tcl interpreter immediately stops processing commands and returns to its caller. If there are several nested invocations of the Tcl interpreter in progress, then each nested command will usually return the error to its caller, until eventually the error is reported to the top-level application code. The application will then display the error message for the user.

In a few cases, some commands will handle certain "error" conditions themselves and not return them upwards. For example, the for command checks for the TCL_BREAK code; if it occurs, then for stops executing the body of the loop and returns TCL_OK to its caller. The for command also handles TCL_CONTINUE codes and the procedure interpreter handles TCL_RETURN codes. The catch command allows Tcl programs to catch errors and handle them without aborting command interpretation any further.

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