Error message not being wrapped to the right length

[wilfwilson]

This is happening for me in the master branch, on a Mac, with readline. It seems as if the output of Error is being wrapped as if it didn't have the 7 characters of "Error, " prepended to it:

$ bin/gap.sh -A -r
 ┌───────┐   GAP 4.12dev-909-gca1c659 built on 2021-06-04 10:33:48+0100
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-apple-darwin20.4.0-default64-kv8
 Configuration:  gmp 6.2.1, GASMAN, readline
 Loading the library and packages ...
 Packages:   GAPDoc 1.dev, PrimGrp 3.4.1, SmallGrp 1.4.2, TransGrp 3.3
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
gap> str := "this is an error message that going to be more than 80 character long although my terminal is only 80 characters wide";
"this is an error message that going to be more than 80 character long althoug\
h my terminal is only 80 characters wide"
gap> Length(str);
117

So the string itself wrapped as I would expect. But:

gap> Error(str);
Error, this is an error message that going to be more than 80 character long although\
 my terminal is only 80 characters wide called from
not in any function at *stdin*:8

which is wrapped too long. It seems like this only affects the first line:

gap> str := "this is an error message that going to be more than 160 character long although my terminal is only 80 characters wide. I will do this by adding some nonsense to the end";
"this is an error message that going to be more than 160 character long althou\
gh my terminal is only 80 characters wide. I will do this by adding some nonse\
nse to the end"
gap> Length(str);
169
gap> Error(str);
Error, this is an error message that going to be more than 160 character long althoug\
h my terminal is only 80 characters wide. I will do this by adding some nonsen\
se to the end called from
not in any function at *stdin*:15

Also...

I'm not sure if this is exactly related, but if I add this example to a test file, then GAP happily accepts the correct behaviour. Contents of test file:

gap> str := "this is an error message that going to be more than 80 character long although my terminal is only 80 characters wide";;
gap> Length(str);
117
gap> Error(str);
Error, this is an error message that going to be more than 80 character long a\
lthough my terminal is only 80 characters wide

Result of test:

gap> Test("width.tst");
true

But if I break the error message then the suggested correct version looks very weird. Contents of test file (last word changed):

gap> str := "this is an error message that going to be more than 80 character long although my terminal is only 80 characters wide";;
gap> Length(str);
117
gap> Error(str);
Error, this is an error message that going to be more than 80 character long a\
lthough my terminal is only 80 characters tall

Result of test:

gap> Test("width.tst");
########> Diff in width.tst:4
# Input is:
Error(str);
# Expected output:
Error, this is an error message that going to be more than 80 charac\
ter long a\
lthough my terminal is only 80 characters tall
# But found:
Error, this is an error message that going to be more than 80 charac\
ter long a\
lthough my terminal is only 80 characters wide
########
false

(As always: maybe I am just doing something 'wrong'?)

[fingolfin]

There are (at least) two issues involved here. One I recently described in this PR comment.

The other is that flushing the output via \c disturbs how we track how many characters where already printed. Indeed, continuing your example:

gap> Print(str, "\c", str, "\n");
this is an error message that going to be more than 80 character long although\
 my terminal is only 80 characters widethis is an error message that going to be more than 80 character long although\
 my terminal is only 80 characters wide

So here first your string is output, then we flush the output buffer. Now GAP thinks we are at the left of the output line and so can output another 80 chars before a line wrap, but that's of course wrong...

One way to deal with this might be to keep track of the cursor position in a separate global var. Another might be to teach GAP how to query the terminal for the cursor position (but I think getting this right cross-platform is tricky, and may involving using terminfo or ncurses or so)