OS.FileSys.mkDir that does not raise exception when the directory already exists - sml

Question

I am trying to write a function that creates a directory but does not raise an error when the directory already exists. This is the function:
fun ensureDir s =
(OS.FileSys.mkDir s)
handle OS.SysErr (_, SOME Posix.Error.exist) => ()
I based the pattern OS.SysErr (_, SOME Posix.Error.exist) on the fact that OS.FileSys.mkDir fails with the following error message when the directory already exists:
Poly/ML:
Exception- SysErr ("File exists", SOME EEXIST) raised
SML/NJ:
uncaught exception SysErr [SysErr: File exists [exist]]
raised at: <mkdir.c>
However, I get this error when I try to define the function in the Poly/ML shell:
poly: : error: qualified name Posix.Error.exist illegal here
Static Errors
This is the error in SML/NJ:
Error: variable found where constructor is required: Posix.Error.exist
What mistake did I make in the function definition?
(Poly/ML 5.7.1; SML/NJ 110.79; Ubuntu 20.04)

Answer 1

Posix.Error.exist is a val and not a constructor (like SOME) so
you cannot use it as a pattern in a pattern matching construct. Here
is an equivalent code for what you intended to do.
fun ensureDir s =
(OS.FileSys.mkDir s)
handle e as (OS.SysErr (_, SOME err)) =>
if err = Posix.Error.exist
then ()
else raise e
;