How to change script dir inside a function?

Question

I try to change the directory inside a bash-script function, but it does not have any effect. Possibly, because the function is run inside a separate process...

Here is a simple script without a function call. It works perfectly:

#!/bin/bash

# Doing it without the function works:
echo "Dir: $PWD" # Output: "/Users/philipp/"
cd ~/Documents
echo "Dir: $PWD" # Output: "/Users/philipp/Documents"

However, once I move the cd into a function, it does not change the directory for the rest of the shell script anymore:

#!/bin/bash

show_or_run() {
    if [[ "yes" = $RUN ]]; then
        eval "$@"
    else
        echo "   $ $@"
    fi
}

echo "Dir: $PWD" # Output: "/Users/philipp/"
show_or_run cd ~/Documents
echo "Dir: $PWD" # Output: "/Users/philipp/", but expected "/Users/philipp/Documents"