On Wed, Dec 11, 2019 at 1:28 AM Henri Menke <henrimenke@gmail.com> wrote:

> In this case I wanted to generate the thin space (\,).  My expectation
> (which worked elsewhere) was that full expansion will turn "\string\\,"
> into "\\," where the two backslashes have catcode 12 and will not be
> expanded further.  Then the Lua string will turn the escape sequence \\
> into a single backslash, therefore ending up with \, in the Lua string.
>
>


\directlua{token.set_macro("foo","\relax")}
(gdb) x/7c str
0x555559376c18: 13 '\r' 101 'e' 108 'l' 97 'a'  120 'x' 32 ' '  0 '\000'
(gdb) p lstr
$24 = 6
where \foo is
 \foo=macro:
->#!.

vs
\directlua{token.set_macro("foo",[==[\relax]==])}
(gdb) x/8c str
0x555559376ea8: 92 '\\' 114 'r' 101 'e' 108 'l' 97 'a'  120 'x' 32 ' '  0
'\000'
(gdb) p lstr
$25 = 7
\foo=macro:
->\relax .

Another example is
\directlua{token.set_macro("foo","\medskip")}\show\foo
which gives the error:   invalid escape sequence near '"^^Kskip \m'.
vs
\directlua{token.set_macro("foo",[==[\medskip]==])}\show\foo
 which gives
\foo=macro:
->\vskip \medskipamount .

Your example:
\directlua{token.set_macro("foo","\string\\,")}\show\foo
gives
(gdb) x/3c str
0x5555593765a8: 92 '\\' 44 ','  0 '\000'
(gdb) p lstr
$29 = 2
 \foo=macro:
->BAD,.

vs
\directlua{token.set_macro("foo",[==[\,]==])}\show\foo
(gdb) x/20c str
0x555559376908: 92 '\\' 109 'm' 115 's' 107 'k' 105 'i' 112 'p' 32 ' '  92
'\\'
0x555559376910: 116 't' 104 'h' 105 'i' 110 'n' 109 'm' 117 'u' 115 's' 107
'k'
0x555559376918: 105 'i' 112 'p' 32 ' '  0 '\000'
(gdb) p lstr
$31 = 19
 \foo=macro:
->\mskip \thinmuskip .


-- 
luigi