suggestions for two very minor improvements to gp user interface

[Kevin Buzzard <buzzard@ic.ac.uk>]

Hi pari-users. Sorry for a long posting on such minor matters: I am
suggesting two very minor tweaks to the gp interface which would make my
life easier, but which I certainly wouldn't be able to program myself,
so I thought I'd tempt others to do so...

The first one: I occasionally use gp to manipulate matrices / power series
with big coefficients, "big" as in "contains 100,000 digit numbers".
Occasionally I miss a semi-colon at the end of a line and suddenly get a
printout of what I don't want to see, namely pages and pages of digits.
I'm sure I'm not the only person this has happened to. This causes great
irritation, because these kinds of printouts inevitably completely fill
up my scrollback memory and hence I lose by far the most efficient method
I have for looking at back at what I have just done recently.

It seems to me hence that if gp finds that it wants to print out something
more than 100K long, there is a chance that the user has made a slip---could
the user perhaps not really want to look at all of such a long output?  So
why not have a flag, disabled by default, called something like
"print_overflow_warning", which, if set to 1, triggers a warning whenever
a huge output is about to appear: just some kind of "answer seems very long,
do you really want me to print it (yes/no)?" prompt would be great.
This would be very easy to do if, when gp is about to print out the
result of calulation %107, the pari output is all 'collected up' and
then printed out in one go---I don't know whether this is the case but
thought that there was a chance that it might be. Note that it should
definitely be disabled by default because it might well break scripts
written to compute a large amount of data and then save it to disk.

The second thing is two little tweaks to readline that are implemented
in MAGMA and are a *lovely* feature. I am just amazed how simple and yet
effective these ideas are. By default, if I am using readline, and I hit
up-arrow, this takes me to the last line I typed in. In MAGMA I can do
more: If I type "abc" at the prompt and _then_ hit up-arrow, it takes me
to the last line I typed that began with "abc". This is very useful if you
want to find that long series of commands that you typed a while ago and
is much more natural than using ctrl-R etc. But it gets even better: once
I've found this line, if I hit return and then down-arrow, it takes me to
the line after the "abc..." line, so if I am looking to do a multi-line
set of commands over again, I only have to search once. As a concrete
example of what I'm trying to explain, take a look at this sample session:

a=somecomplicatedfunction();
b=a^4+a^2+1;
c=log(b)+2;
print(c^2/2)

(lots of lines of gp input omitted)

(now decide to try another value of a)

a=someothercomplicatedfunction();

(now I realise that I want to compute c as above, just like before).

In Pari I would now hit something like Ctrl-R b=(enter) Ctrl-R c=(enter)
Ctrl-R print Ctrl-R Ctrl-R Ctrl-R... (until I've found the print statement
I want!) etc etc etc.

In Magma I would type

b=(up-arrow) (enter) (down-arrow) (enter) (down-arrow) (enter) and that
would be the end of it.

It's perhaps hard to appreciate how nice this feature is until you've seen
it in action, but having used MAGMA a fair amount, I now realise I really
miss this feature in pari. Is this kind of thing possible to implement in
gp or would it involve messing with readline?

Kevin