How to calculate factorial with xfp package

Question

I'm trying to calculate the factorial with the xfp package. In the documentation (http://ctan.cs.uu.nl/macros/latex/contrib/l3packages/xfp.pdf), I have found the function name to be fact. My code looks as follows:

\documentclass{article}
\usepackage{xfp}
\begin{document}
\fpeval{fact(4)}
\end{document}

But, I get following error:

! Undefined control sequence.
<argument> \LaTeX3 error: 
                           Unknown fp word fact.
l.4 \fpeval{fact(4)}

While other functions do work (e.g. no error for \fpeval{16*4^3} or \fpeval{cosd(60)*45} )

What am I doing wrong? I am using TeXstudio.

UPDATE

In the preamble I set \usepackage{xfp}[2019/08/25] with the date I found in the documentation between square brackets.

I get the same error as before and following warning:

You have requested, on input line 3, version `2019/08/25' of package xfp, 
but only version `2018-09-24 L3 Floating point unit' is available.

When I set \usepackage{xfp}[2018/09/24], I get same error (and no warning).

In any case (with or without date in square brackets), I see following lines in log-file:

Package: expl3 2018-09-24 L3 programming layer (code)
Package: xparse 2018-09-24 L3 Experimental document command parser
Package: xfp 2018-09-24 L3 Floating point unit

I'm using Texworks remotely via the company server, so I have no access to package manager or whatever. Also can not use texdoc. Is there a way to force package update from within Texworks? (probably not ...)

Answer 1

You should really update your L3 packages. But, if this is for some reason not desired, then you can copy the relevant code from the updated package in your document in order to use the new code.

MWE:

\documentclass{article}
\usepackage{xfp}
\makeatletter
\ExplSyntaxOn
\long \def \__kernel_primitive:NN #1#2
    {
      \tex_global:D \tex_let:D #2 #1
    }
\__kernel_primitive:NN \numexpr               \tex_numexpr:D
\cs_new_eq:NN \int_value:w      \tex_number:D
\cs_new_eq:NN \__fp_int_eval:w \tex_numexpr:D
\cs_new:Npn \__fp_parse_word_fact:N
  { \__fp_parse_unary_function:NNN \__fp_fact_o:w ? }
\int_const:Nn \c__fp_fact_max_arg_int { 3248 }
\cs_new:Npn \__fp_fact_o:w #1 \s__fp \__fp_chk:w #2#3#4; @
  {
    \if_case:w #2 \exp_stop_f:
      \__fp_case_return_o:Nw \c_one_fp
    \or:
    \or:
      \if_meaning:w 0 #3
        \exp_after:wN \__fp_case_return_same_o:w
      \fi:
    \or:
      \__fp_case_return_same_o:w
    \fi:
    \if_meaning:w 2 #3
      \__fp_case_use:nw { \__fp_invalid_operation_o:fw { fact } }
    \fi:
    \__fp_fact_pos_o:w
    \s__fp \__fp_chk:w #2 #3 #4 ;
  }
\cs_new:Npn \__fp_fact_pos_o:w #1;
  {
    \__fp_small_int:wTF #1;
      { \__fp_fact_int_o:n }
      { \__fp_invalid_operation_o:fw { fact } #1; }
  }
\cs_new:Npn \__fp_fact_int_o:n #1
  {
    \if_int_compare:w #1 > \c__fp_fact_max_arg_int
      \__fp_case_return:nw
        {
          \exp_after:wN \exp_after:wN \exp_after:wN \__fp_overflow:w
          \exp_after:wN \c_inf_fp
        }
    \fi:
    \exp_after:wN \__fp_sanitize:Nw
    \exp_after:wN 0
    \int_value:w \tex_numexpr:D
    \__fp_fact_loop_o:w #1 . 4 , { 1 } { } { } { } { } { } ;
  }
\cs_new:Npn \__fp_fact_loop_o:w #1 . #2 ;
  {
    \if_int_compare:w #1 < 12 \exp_stop_f:
      \__fp_fact_small_o:w #1
    \fi:
    \exp_after:wN \__fp_ep_mul:wwwwn
    \exp_after:wN 4 \exp_after:wN ,
    \exp_after:wN { \int_value:w \__fp_int_eval:w #1 * (#1 - 1) }
    { } { } { } { } { } ;
    #2 ;
    {
      \exp_after:wN \__fp_fact_loop_o:w
      \int_value:w \__fp_int_eval:w #1 - 2 .
    }
  }
\cs_new:Npn \__fp_fact_small_o:w #1 \fi: #2 ; #3 ; #4
  {
    \fi:
    \exp_after:wN \__fp_ep_mul:wwwwn
    \exp_after:wN 4 \exp_after:wN ,
    \exp_after:wN
      {
        \int_value:w
        \if_case:w #1 \exp_stop_f:
        1 \or: 1 \or: 2 \or: 6 \or: 24 \or: 120 \or: 720 \or: 5040
        \or: 40320 \or: 362880 \or: 3628800 \or: 39916800
        \fi:
      } { } { } { } { } { } ;
    #3 ;
    \__fp_ep_to_float_o:wwN 0
  }
\ExplSyntaxOff
\makeatother
\begin{document}
\fpeval{fact(4)}
\end{document}

Note that this may break at some point, I tested a few cases but there might be code that I didn't copy that is called for other cases that I didn't test. So update if you can.