Syntax errors in EvalString sometimes not displayed correctly

[zickgraf]

Observed behaviour

gap> EvalString( "(1;" );
Syntax error: ) expected in stream:1
�
Error, Could not evaluate string.

Expected behaviour

gap> EvalString( "(1;" );
Syntax error: ) expected in stream:1
_EVALSTRINGTMP:=(1;
                  ^
Error, Could not evaluate string.

Sometimes the error is displayed as expected:

gap> EvalString( ";;" );
Syntax error: expression expected in stream:1
_EVALSTRINGTMP:=;;;
                ^^
Error, Could not evaluate string.

but I could not find a pattern.

Copy and paste GAP banner (to tell us about your setup)

 ┌───────┐   GAP 4.12.1 of 2022-10-20
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64-kv8
 Configuration:  gmp 6.2.1, GASMAN, readline
 Loading the library and packages ...
 Packages:   GAPDoc 1.6.6.dev, IO 4.7.0dev, PrimGrp 3.4.2, SmallGrp 1.5, TransGrp 3.6.3
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'

[fingolfin]

The funny character is ASCII 255 and this comes from reading till EOF. We've had similar buglets in the past. Not sure if it's worth the effort to fix it properly, though? What's the usecase?

[zickgraf]

What's the usecase?

I'm using EvalString for metaprogramming and it's much easier to find errors if the error message points right at the point where something is missing/unexpected. The above is only a minimal example, this also happens in more complex cases.

[fingolfin]

Note that you can avoid the issue (at least in the cases I am aware of) by ensuring your string always ends with a newline:

gap> EvalString( "(1;\n" );
Syntax error: ) expected in stream:1
_EVALSTRINGTMP:=(1;
                  ^
Error, Could not evaluate string.

[zickgraf]

A newline in the input string is not always sufficient:

gap> EvalString( "(1\n" );
Syntax error: ) expected in stream:2
�
Error, Could not evaluate string.

But indeed a newline helps when adding it after the semicolon added by EvalString. This also explains why it usually does not happen on the REPL (commands are naturally completed by a newline) or when reading from files (which usually end with a newline character). As expected, the same problem occurs when ommiting the newline on the REPL:

$ echo -n "(1;" | gap
 ┌───────┐   GAP 4.12.1 of 2022-10-20
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64-kv8
 Configuration:  gmp 6.2.1, GASMAN
 Loading the library and packages ...
 Packages:   GAPDoc 1.6.6.dev, IO 4.7.0dev, PrimGrp 3.4.2, SmallGrp 1.5, 
             TransGrp 3.6.3
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
gap> > Syntax error: ) expected
�

I will create a PR.