TIP #329 Version 1.9: Try/Catch/Finally syntax

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes the addition of new core commands to improve the exception handling mechanism. It supercedes TIP #89 by providing support for the error options dictionary introduced in Tcl 8.5 by TIP #90.

Rationale

See TIP #89 for general rationale for enhancing exception handling.

The try syntax presented here is not intended to replace catch, but to simplify the expression of existing exception/error handling techniques, leading to greater code clarity and less error-prone workarounds for finally blocks. There is no deficiency in the functionality of Tcl's exception handling mechanisms - what is lacking is a more readable syntax and a standard for behaviour across packages for the common case of catching a subset errors or exceptions that are thrown from within a particular block of code.

In Tcl 8.4 exceptions could be caught using catch, and exception information was available via the catch return value and resultvar. If the return value was TCL_ERROR (1) then the globals ::errorCode and ::errorInfo would be set according to the exception raised. TIP #89 was written to work with this model, such that a catch handler (in a try...catch) would be able to capture the resultvar, errorCode and errorInfo.

Tcl 8.5 implements TIP #90 which extends catch to allow an additional dictionary of options (error information) to be captured. These options supercede the ::errorInfo and ::errorCode globals (though those are still supported for backward compatibility). It is therefore logical to extend/correct the syntax of TIP #89 to support the options dictionary in preference to the older mechanism for capturing exception information.

Benefits of adding this functionality to the core:

• Bring to Tcl a construct commonly understood and