In a prior post I had illustrated the
use of the Tcl promise package for asynchronous computing with
some examples. There we had ignored the possibility of errors and
exceptions and how they are handled in promise-based code. In
this post, we build on the examples in that post to illustrate how
promises greatly simplify handling of errors in async code.
I had previously described an experimental
implementation of promises for Tcl. On re-reading my earlier
post, I was somewhat dissatisfied with the treatment there in that
I did not feel it fully reflected the value of the promise
abstraction, getting somewhat caught up in the details. This post
takes a somewhat different approach, concentrating more on
examples and refraining from going into detail about each command
or method. Here I am more interested giving you a flavor of programming
with promises and motivating you to explore further.
This post is obsoleted by the promise
package (based on the code is this post) and by
on the topic. Nevertheless it may still hold some tutorial benefit.
the promise abstraction for asynchronous code. Implemented in
various forms in third party libraries, it proved sufficiently
useful to be formally defined and incorporated into ECMAScript 6.
Other languages, from Python and Scala to C#, C++ and Java, have
implementations of promises in various flavors
(sometimes separated into promises and futures). Not finding one
for Tcl (we cannot let Tcl lag!),
I started on an experimental implementation described in this post.
Tcl has some commands that are undocumented because they
are liable to change, or even be removed, at any time,
even in a patch release. Nevertheless, these commands can be
very useful in exploring and understanding the inner workings
of Tcl and in some cases, dealing with issues related to performance
or interaction with external systems like COM on Windows.
Tcl 8.6 shipped with the Tcl Database Connectivity (TDBC) API for accessing
disparate SQL database implementations in a standard manner. I happened
to need it for the first time recently and collected my notes
into an introductory article
for the benefit of those who, like me, prefer to be spoon fed.
There is no shortage of hotkey programs for Windows, many of them of high
quality. And of course Windows itself allows you to define hotkeys.
However, a hotkey program in Tcl is not only very simple to write,
it offers the full flexibility and power of Tcl
behind it. Meaning what exactly? Read on.
There are times during software development when you want to run
in interactive mode with
maximum privileges on a system, be God as it were. One might think
running as Administrator would do it but it doesn't.
To be truly omnipotent
on Windows, you have to run under the LocalSystem
It is easy enough with Tcl and this post shows you how.
As a side bonus, it also describes how to inject processes into the interactive
user's desktop to run under the user's account as well.
One of the strengths of Tcl
is the ease of integration with other software, whether they
be COM components, libraries or even executable programs that
are not designed for interaction with other programs. Here we look
the facilities Tcl offers related to the last of these --
running external programs and optionally
interacting with them using standard I/O mechanisms.
Building the Tcl TLS extension on Unix/Linux is straightforward
because OpenSSL libraries are already present and installed. It's a
different matter on Windows where you need to build OpenSSL yourself.
This short article describes the process.
As for many pieces of software, one of the issues in testing
TWAPI is the number of different platform
configurations under which the test suites need to be run. Manual
configuration is tedious and error-prone so automation of the process
using VMware and the vix package is a big win.