“new” and “set” in LaTeX3

Question

I have some questions in LaTeX， There are many "new" and "set" in LaTeX3， I don't quite understand when “new” is needed，such as

\int_set:Nn \l_my_int{12}

An error occurred

! Undefined control sequence.
<argument> \l_my_int                                 
l.11 \int_set:Nn \l_my_int {12}

also，i use

\tl_set:Nn \l_my_tl{\maketitle}

it is normal

Hope to explain to me，thanks！

Read section 4.3.1 "defining functionses" in interface3.pdf. — user202729, Dec 13 '21 at 04:45
Also you can't quite rely on doing wrong things will absolutely raise error. See https://tex.stackexchange.com/questions/270296/error-interception#comment649128_270296 . (actually expl3 has some debug mode or something, didn't try it) — user202729, Dec 13 '21 at 05:00
See also latex3 - When should one use a local or global variable in LaTeX? - TeX - LaTeX Stack Exchange — user202729, Dec 13 '21 at 05:08

score 10 · Answer 1 · answered Dec 13 '21 at 07:51

In expl3, all variables should be declared before use, as this is the convention for the language. At the implementation level, some expl3 variables are TeX registers, whereas others are TeX macros. The latter can be created without allocation, but registers need to be allocated before use (at least using a name).

There is checking code available that will pick up if you 'shortcut' and forget to declare a variable before assigning it, even if behind the scenes it's a macro. However, as that slows things down, it's not active by default. You can active it with

\RequirePackage[enable-debug,check-declarations]{expl3}

“new” and “set” in LaTeX3

1 Answers1