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Re: Request to abstract write operations to stdout and stderr in jitter

From: Luca Saiu
Subject: Re: Request to abstract write operations to stdout and stderr in jitter
Date: Thu, 12 Nov 2020 12:58:48 +0100
User-agent: Gnus (Gnus v5.13), GNU Emacs 27.0.50, x86_64-pc-linux-gnu

Hello Mohammad.

On 2020-11-12 at 11:07 +0330, Mohammad-Reza Nabipoor wrote:

> What about a callback printer:
> ```c
> jitter_print_context ctx = jitter_print_context_make_callback (clbk, 
> clbkdata);
> ```
> Every time jitter wants to print something, the callback `clbk` gets called.
> For example, after calling `jitter_print_int (ctx, 10, 0)`, the `clbk` will
> be called like this:
> ```c
> clbk(JITTER_PRINT_TOKEN_INT, "10", clbkdata);
> ```
> [...]

I am not fond of this specific API, the reason being that in practice it
is not mandatory to print by specifying a token name (actually class or

Think for example of JitterLisp, which should be a typical use case:
JitterLisp is an interactive Lisp system, distributed along with Jitter
as an advanced example.  Even if JitterLisp has a few interesting ideas
from the point of view of output it is very ordinary: it will print a
large amount of ordinary text (banners, prompts, warning, time
measurements, maybe in the future interactive help, lone newline
characters, "Goodbye" at the end), some Lisp objects (for example when
showing the result of an expression typed by the user at the REPL), and
occasionally also VM routines and disassemblies.  Once the transition is
over the entire output of JitterLisp will be printed out through print

What I want to stress is that a large part of the output is *not*
explicitly styled, and I believe that should remain the case.

However your example raises an important point that I will deal with: a
user-specified function should have access to the current class and URL.
This is currently not the case but the feature would be useful, and easy
to support in a clean way.  Already at the present time any Jitter print
context internally keeps track of the current class (maintaining its own
stack of nested class activations) and URL.  Apart from the fact that
either can be NULL at any point during printing, user functions should
have access to this information, as in your example.

What I am planning in practice is simply to add parameters (and possibly
to require a longer lifetime from user-supplied URL data compared to
what the comments are saying now; but I might also copy strings into my
own memory) to the functions supplied by the user within struct
jitter_print_context_kind_private , which is similar to poke's struct
pk_term_if .  Predefined functions intended for the user to call such as
jitter_print_int or jitter_print_char_star would call user-supplied
functions passing these new parameters as well.

You have convinced me about this, and I will tentatively implement this
change in the branch.  Of course a user will always be able to ignore
any parameter she is not interested in, in her own functions.  No
particular action is needed for ensuring that those parameters have the
right values, other than correctly nesting calls to
jitter_print_begin_class , jitter_print_end_class ,
jitter_print_begin_hyperlink , jitter_print_end_hyperlink .

Now a question for you.  Would it be useful to add, in addition to
classes and URLs, other user-specified data, which might nest in a stack
fashion like classes do?  This can be done, again, with the caveat that
not all text will always be decorated -- some user code, such as a Poke
object printer, may tirelessly style and annotate every single character
in the output, but even that might break once the Poke object printer
prints, for example, a closure containing PVM code: if you call a VM
routine printer you pass control to my functions, which may leave holes
in the otherwise uninterrupted sequence of metadata you desire.

I would also add functions to inquire, giving a print context, about the
current class and URL -- and user data as well, if added.  Those
function would be callable by the user at any time when a print context

In fact it would be possible, if the user could choose multiple data
kinds of her own (with user-specified nestability: for example classes
are nestable, URLs are not), to have classes and URLs just implemented
as instances of user data, possibly predefined.  Let us call all of
those decorations just "metadata".

What do you think?  Now for the first time since starting this task I am
having a definite impression of a "good design", instead of the
instinctive dislike I feel for a trivial wrapper.  Each metadata kind
would nest, or not be allowed to nest, independently from the others.
Libtextstyle classes and URLs would fit this design.  Your token
enumerate would fit.  Indentation would fit as well.

Let me know.


Luca Saiu
* My personal web site:  http://ageinghacker.net
* GNU epsilon:           http://www.gnu.org/software/epsilon
* Jitter:                http://ageinghacker.net/projects/jitter

I support everyone's freedom of mocking any opinion or belief, no
matter how deeply held, with open disrespect and the same unrelented
enthusiasm of a toddler who has just learned the word "poo".

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