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Re: Advice on troubleshooting function to "unscroll"

From: Marcin Borkowski
Subject: Re: Advice on troubleshooting function to "unscroll"
Date: Mon, 26 Jan 2015 00:15:53 +0100

On 2015-01-25, at 23:56, Will Monroe <> wrote:

> On 01/25/2015 04:45 PM, Drew Adams wrote:
>> The command `scroll-up' is indeed older, and it is still usable.
>> It is, in fact, the core of the `scroll-up-command' implementation.
> Interesting.  Given what the author's assumptions of how 'scroll-up' 
> should work and how the newer 'scroll-up-command' seems to share some, 
> but not all, of that functionality, it would probably be interesting to 
> see where they differ.  I may not be expressing this clearly, but you've 
> helped to clarify the question I want to answer to be: "why did 
> scroll-up-command work where the still useful but not appropriate 
> scroll-up did not?"

That's why it's useful to say

M-x find-function RET scroll-up-command RET!

>>> I've hesitated from diving into Edebug because there was no
>>> "error" as such.  That is, the code "worked" it just didn't do
>>> what I intended.
>> Using the debugger (and debugging generally) is not necessarily
>> about finding why Emacs raises a particular error.  (That is the
>> particular use of variable `debug-on-error': enter the debugger
>> to show a backtrace when an error is raised.)
>> In addition to Edebug, there is the regular Emacs debugger, aka
>> `debug'.  Some of us prefer to use that.  Others prefer to use
>> it in some cases but `edebug' in other cases.

Thanks, Drew, for mentioning that!

>> Keep in mind too that any debugger does not necessarily tell you
>> everything that goes on, and its representation of what happens
>> with debugging turned off is not flawless.  In particular, this
>> is because using the debugger itself changes what Emacs does.
>> (Think of the debugger's buffer display and its handling of
>> input events, for example.)
>> The regular debugger is what you get with `debug-on-entry' and
>> by inserting calls to `(debug)' in code to serve more or less as
>> breakpoints.  See `C-h f debug' for information about evaluating
>> and displaying the results of sexps (cdr ARGS) upon entry into
>> the debugger.

I do not like the (debug) approach, since it seems to be less
newbie-friendly.  It is true, of course, that Edebug does not always
work as intended.

> This is very helpful stuff to consider as I begin to think about what I 
> want to accomplish when using Edebug or debugger.
> And if I'm being honest about why I did not go to Edebug it's because I 
> was anticipating great difficulty reading the backtrace statements.  I 
> thought my problem might be compounded by a more general illiteracy in 
> that regard...

Now I'm starting to think that it would be nice to study the backtraces
once and for all and write about them (for myself and for others)...

> Thanks again, Drew.  A really helpful answer!

Me too!

> Will


Marcin Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University

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