The call type(e, 'filedesc') checks whether the expression e may be a valid file descriptor. File descriptors are expressions of types integer, string, or symbol.

Note that this type check is a surface type check only. It does not verify that the expression e actually refers to an open file.

$\mathrm{type}\left(\mathrm{book}\,'\mathrm{filedesc}'\right)$

$\mathrm{true}$

$\mathrm{type}\left(12\,'\mathrm{filedesc}'\right)$

$\mathrm{true}$

$\mathrm{type}\left(12x\,'\mathrm{filedesc}'\right)$

$\mathrm{false}$

$\mathrm{type}\left(''12*x''\,'\mathrm{filedesc}'\right)$

$\mathrm{true}$

$\mathrm{type}\left(\mathrm{book}\left[1\right]\,'\mathrm{filedesc}'\right)$

$\mathrm{false}$