The variable $? does not work on $PS1 on mksh

Question

I want my prompt to show the exit status of the last command, so I set my PS1 to this:

PS1="$? > "

But it always prints 0 >.
Even when I run false, for example, the prompt does not prints 1 > or whatever the exit status is.

Why does this occur?

EDIT:

I tried to use a function to set my prompt, testing whether the exit status was greater than 0, so it will not print 0 > always, only when the exit status is nonzero.

 promptcmd() {
    _EXIT=$?
    test $_EXIT -gt 0  && printf "\e[1;31m [$_EXIT]"
    printf "\e[0m ❯ "
    unset _EXIT
 }

 PS1="$(promptcmd)"

But it also does not work.

Answer 1

$? was expanded when you defined PS1, because you used double quotes.

You can use single quotes to defer expansion until PS1 is displayed:

PS1='$? > '

This kind of "double expansion" is not a property of parameters in general, but a result of how the shell uses the value of PS1. echo "$PS1" will still show the literal string $? >, but when the shell displays the prompt, it will expand any parameter expansions found in the value.