TIP:            348
Title:          Substituted 'errorstack' / 'traceback'
Version:        $Revision: 1.17 $
Author:         Alexandre Ferrieux <alexandre.ferrieux@gmail.com>
State:          Final
Type:           Project
Vote:           Done
Created:        26-Feb-2009
Post-History:   
Keywords:       Tcl,debugging
Tcl-Version:    8.6

~ Abstract

This TIP proposes to add an '''errorstack''' options dict entry and associated
'''info''' subcommand, giving a "substituted" traceback similar to Python's or
gdb's ones.

~ Background

The '''::errorInfo''' variable is a valuable tool for debugging; however, it
yields mostly static information regarding each level of procedure call, as it
only gives the static text (extracted from the source) at the call site. This
leads to frustrating situations, like when an error occurs at a deep level of
a recursive function, '''::errorInfo''' repeatedly reporting "f $x [[foo
[[bar]]]]" or similar un-substituted commands. In other languages, the
traceback is more useful in that it contains the actual values passed as
arguments.

~ Proposed Change

This TIP proposes to create an '''-errorstack''' options dictionary entry, and
an associated '''info errorstack''' ?''interp''? command returning a list
containing the [['''info level''' 0]] lists of command-and-args at each
stack frame level at the time of error unwinding.

~ Rationale

In a natural implementation, its construction is analogous to that of
'''::errorInfo''', which is built incrementally, one level at a time while
popping back from the error site.  The only differences are that:

 1. dynamic arglists (from [['''info level''' 0]]) are stored,

 2. the result is a true list built by (the equivalent of) '''lappend''', and

 3. the granularity is coarser than with '''::errorInfo''' since there is just
    one element per stack frame level (and not for intervening '''while''' or
    '''foreach''' constructs) and no adornment like "... while executing ..."

Measurements show that the performance hit of maintaining the error stack is
small, being under 5% for realistic error stack sizes, and hits only the error
cases. To achieve this speed, the list-growing is done carefully, reusing
earlier optimizations made on list operations for the in-place case, so that
in routine use the allocated Tcl_Obj, internal representation, and element
array storage backing the list never change. This way, even code heavily
relying on '''catch''' across dozens of stack frame levels won't noticeably
suffer from the error stack machinery.

~ Definition

The error stack is an even-sized list alternating tokens and parameters.
Tokens are currently either '''CALL''' or '''UP''', but other values may be
introduced in the future; '''CALL''' indicates a procedure call, and its
parameter is the corresponding [['''info level''' 0]]; '''UP'''' indicates a
shift in variable frames generated by '''uplevel''' or similar, and applies to
the previous '''CALL''' item:

 > CALL {foo a} UP 1 CALL {bar b} CALL {baz c} UP 2 CALL {gnu d}
   CALL {gnats e}

The above value indicates that [[foo a]] was called by [[bar b]] through
[[uplevel 1]]. Similarly, [[baz c]] was itself lifted to toplevel by an
[[uplevel 2]] in [[gnu d]].

~ Reference Implementation

The reference implementation can be found on
SourceForge[https://sourceforge.net/support/tracker.php?aid=2868499].

An earlier, extremely slow, pure-Tcl proof of concept based on write traces on
'''::errorInfo''' can be found on the Wiki[http://wiki.tcl.tk/traceback].

~ Copyright

This document has been placed in the public domain.