Problem with evaluating integers with LaTeX3 with an intermediate variable

Question

I have two codes for evaluating number. First of it for evaluating floating point number:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\evaluate}{m}
 { \fp_set:Nn \l_myvar_fp {#1}
   \fp_eval:n { \l_myvar_fp }

 }
\ExplSyntaxOff
\begin{document}

\evaluate{2+3}

\end{document}

It is well done and I get correct answer.

Then of it for evaluating integer number:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\evaluate}{m}
 { \int_set:Nn \l_myvar_int {#1}
   \int_eval:n { \l_myvar_int }

 }
\ExplSyntaxOff
\begin{document}

\evaluate{2+3}

\end{document}

In the second example I just change fp to int, but I get an error in the second case:

Undefined control sequence

But if I directly use \int_eval:n without using an intermediate variable \l_myvar_int it is well done.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\evaluate}{m}
 { \int_eval:n { #1 }

 }
\ExplSyntaxOff
\begin{document}

\evaluate{2+3}

\end{document}

Only after definition variable via \int_new:N \l_myvar_int it is successfully compiled.

Why in first case code well done without definition of a new variable, but it the second case does not?

Answer 1

You've not declared your variables. Due to the underlying implementation detail, some variable types may allow you to set them without declaration. However, this is not something that is supported (as in we reserve the right to change things). You can force expl3 to check declarations, thus

\documentclass{article}
\usepackage[check-declarations]{expl3}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\evaluate}{m}
 { \fp_set:Nn \l_myvar_fp {#1}
   \fp_eval:n { \l_myvar_fp }

 }
\ExplSyntaxOff
\begin{document}

\evaluate{2+3}

\end{document}

will yield

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! LaTeX error: "check/non-declared-variable"
! 
! The variable \l_myvar_fp has not been declared on line 14.
! 
! See the LaTeX3 documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

Checking is not active as-standard largely for performance reasons: for data types implemented as macros, checking needs extra macro code.

For development work, using

\usepackage[check-declarations]{expl3}

during testing is recommended, though in 'production' code one would normally omit the option.