Mfrhtyj

I have a macro that uses prg_replicate:nn to produce a number of spaces, then pass those spaces, after f-expanding prg_replicate:nn, to another macro. The problem is that when I use exp_args:Nf, it will use the romannumeral 0 thingy which can consume a space. Guess what, this space is removed from my replicated spaces, so after the whole thing is completed I have one space less.

The issue is reproduced with this MWE:

documentclass{article}

usepackage{expl3}

begin{document}

ExplSyntaxOn

cs_new:Npn __printf_pre_output:n #1 { [ tl_to_str:n {#1} ] }

cs_set:Npn __printf_tmp:w #1

  {

    cs_new:Npn __printf_format_final_print:n ##1

      {

        exp_args:Nf __printf_pre_output:n

          { prg_replicate:nn {##1} {#1} }

      }

  }

__printf_tmp:w { ~ }

ttfamily

Wrong,~4~spaces:~__printf_format_final_print:n {5}par

Right,~5~spaces:~[     ]par

ExplSyntaxOff

end{document}

which produces:

I couldn't find a way to stop romannumeral from eating one of my spaces besides the obvious "do the whole thing with an extra space to begin with".

For the time being I'm doing the replication with an extra space to feed to romannumeral, but it seems a hackish way to do that. Is there another possibility?

I would simply use the new e-type approach here (i.e. using the expanded primitive or equivalent):

documentclass{article}

usepackage{expl3}

begin{document}

ExplSyntaxOn

cs_new:Npn __printf_pre_output:n #1 { [ tl_to_str:n {#1} ] }

cs_set:Npn __printf_tmp:w #1

  {

    cs_new:Npn __printf_format_final_print:n ##1

      {

        exp_args:Ne __printf_pre_output:n

          { prg_replicate:nn {##1} {#1} }

      }

  }

__printf_tmp:w { ~ }

ttfamily

Wrong,~4~spaces:~__printf_format_final_print:n {5}par

Right,~5~spaces:~[     ]par

ExplSyntaxOff

end{document}

This will likely become the standard approach for such cases: f-type expansion is much less important now that we have e-type either directly using the expanded primitive or (where necessary) emulated. Note that MiKTeX already has expanded in pdfTeX and XeTeX, and that this will appear in TeX Live 2019. Also note that LuaTeX has had expanded from day one. (Emulation is reliable but does have a performance impact: at present, it probably is best avoided for any code which needs to work in tight loops or similar.)

Like @jfbu mentioned, expanding twice is enough for prg_replicate:nn. So using exp_args:No twice on it suffices:

documentclass{article}

usepackage{expl3}

begin{document}

ExplSyntaxOn

cs_new:Npn __printf_pre_output:n #1 { [ tl_to_str:n {#1} ] }

cs_set:Npn __printf_tmp:w #1

  {

    cs_new:Npn __printf_format_final_print:n ##1

      {

        exp_args:NNo exp_args:No __printf_pre_output:n

          { prg_replicate:nn {##1} {#1} }

      }

  }

__printf_tmp:w { ~ }

ttfamily

Wrong,~4~spaces:~__printf_format_final_print:n {5}par

Right,~5~spaces:~[     ]par

ExplSyntaxOff

end{document}

I can only second @JosephWright and @Skillmon answers but speaking of what I know, with xint you only need to expand once romannumeralxintreplicate{100}{ } to get 100 spaces. So if inside a macro

defx{romannumeralxintreplicate{100}{ }}

you only need to expand x twice.

Or

defx{xintreplicate{100}{ }}

then you can do romannumeralx.

Oh wait, I completely forgot: romannumeralxintreplicate basically IS (up to different user interface) prg_replicate:nn, I COPIED its underlying implementation ;-).