The function print_preprocess returns the pretty-printed form of an expression, as print would display it.

In particular, procedures of the form `print/foo` are applied to functions foo(a, b, ...) and ModulePrint procedures are applied to modules. This is particularly useful when you write a `print/foo` or ModulePrint procedure and want to display a subobject.

m := module()
  export a;
  local ModulePrint := proc()
    return [print_preprocess(a)];
  end proc;
end module:

$m:-a≔5\:$$\mathrm{eval}\left(m\right)$

$\left[5\right]$

$m:-a≔\mathrm{LinearAlgebra}:-\mathrm{RandomMatrix}\left(20\right)\:$$\mathrm{eval}\left(m\right)$

$\left[\left[\begin{array}{c}\mathrm{20\; x\; 20}\mathrm{Matrix}\\ \mathrm{Data\; Type:}\mathrm{anything}\\ \mathrm{Storage:}\mathrm{rectangular}\\ \mathrm{Order:}\mathrm{Fortran\_order}\end{array}\right]\right]$

`print/foo` := proc()
local oof, i;
  return oof(seq(print_preprocess([_passed][i]), i = _npassed .. 1, -1));
end proc:

$\mathrm{foo}\left(1\,2\right)$

$\mathrm{oof}\left(2\,1\right)$

$m:-a≔\mathrm{foo}\left(1\,2\right)\:$$\mathrm{eval}\left(m\right)$

$\left[\mathrm{oof}\left(2\,1\right)\right]$