PROMPT_COMMAND and PS1 error

[Florian Mayer]

Hello,

I'm currently finishing my work building
my own "prompt-lib". This, of course,
involves manipulating the variables PROMPT_COMMAND and PS{1,2}.
Problem description:
Imagine a new interactive session where you source
one file (first.sh). first.sh also sources another file (second.sh)
which creates a global associative array. The interactive session now 
contains a function
"sl-get" which should emit a "prompt". It echos either "0" or "1"
depending on whether the date (exact to the minute) has changed
since the last call to this function. Now PS1 should get updated every
new command. PROMPT_COMMAND obviously is the variable to use here.
So
$ PROMPT_COMMAND=sl-set-prompt
should do the trick... But, sadly, it doesn't.
Strangely though, the value in "FOO" gets updated if you manually
call sl-get.

For better understanding, I appended the snippets I used to
reproduce the problem.

Further diagnosis information:
Bash version: 4.3.42(1)-release (x86_64-unknown-linux-gnu)
Operating System: Arch linux

==== file: first.sh ====
. second.sh

sl-get(){
    sl-set
    sl-notify-change
    echo $?
}

sl-set-prompt(){
    PS1=$(sl-get)" >>"
}
==== file end ====

==== file: second.sh ====
declare -Ag FOO

FOO=(
    ["data"]=""
    ["oldval"]=""
)

sl-set(){
    FOO["data"]=$(date "+%d.%m.%y-%H:%M")

    return 0
}

sl-notify-change(){
    if [ "${FOO["oldval"]}" != "${FOO["data"]}" ]; then
        FOO["oldval"]=${FOO["data"]}
        return 0
    else
        return 1
    fi
}
==== file end ====

Best regards
Florian Mayer