+
+# Lisp shell
+
+There is [scsh](???), but it's not really what I'm looking for. Using emacs as
+login shell (with the `eshell` package) comes closest to it regarding both
+with existing commands and integration of Lisp-based ones. However, while
+pipes work as expected with `eshell`, data is still passed around as
+(formatted) text. There doesn't seem to be an easy way to pass around
+in-memory objects, at least while staying in Emacs itself. That would of
+course mean to reimplement some (larger?) parts of that system.
+
+This all ties in to the idea that unstructured text isn't the best idea to
+represent data between processes. Even though Unix pipes are extremely useful,
+the ecosystem of shell and C conventions means that the obvious way isn't
+completely correct, meaning that there are edge cases to consider. The best is
+something as innocent as `ls | wc -l`, which will break, depending on the shell
+settings, with some (unlikely) characters in filenames, i.e. newlines.
+
+#
+
+One of the problems is obviously that in order to pass around structured data,
+i.e. objects, all participants have to understand their format. Passing
+references won't work without OS support though.
+
+Instead of having unstructured streams, use streams of (data) objects. The
+distinction here is Plain Old Objects (PODs) instead of objects with an
+associated behaviour.
+
+Let's take a look at standard Unix command line tools (I'm using GNU Coreutils
+here) in order to reproduce the behaviour and/or intent behind them:
+
+## Output of entire files
+
+The first command here is `cat`. Although GNU `cat` includes additional
+transformations, this command concatenates files. Similar to the description,
+we can image a `CAT` to perform a similar operation on streams of objects.
+
+It doesn't make much sense to concatenate a HTML document and an MP3 file
+(hence you won't do it in most cases anyway). However, since files are
+unstructured, `cat` can work on them.
+
+# Registering functionality
+
+Although you can call commands individually on files, some of them form an
+ad-hoc service interface already: The C compiler, along with the toolchain
+forms one such interface, where you're required to use the same interface if
+you want to seamlessly replace one part of the toolchain.
+
+Same goes for the Coreutils: As long as you honour the interface, programs can
+be replaced with different implementations.
+
+# Interactive commands
+
+Emacs has a special form `interactive` to indicate whether a command can be
+directly called via the command prompt. There is also special handling there
+to accomodate the use of interactive arguments. This is something that can be
+generalised to the OS. An example of where this already happens is the
+`.mailcap` file and the `.desktop` files from desktop suites.
+
+# Threading and job control
+
+Unfortunately getting proper job control to work will be a bit of a problem in
+any Lisp implementation, since the best way to implement the concurrent jobs is
+using threads, which are not particularly suited for handling the multitude of
+signals and other problems associated with them. Even without job control
+pipelines implemented in Lisp require shared in-memory communication channels,
+so something like (object-based) streams, mailboxes, or queues are necessary to
+move IO between different threads.
projects / blog.git / commitdiff