How to effectively call a function in another script?

suppose we have a script:

script_a.vim:
" start of script a

let s:str1 = "test1"
let s:str2 = "test2"

function s:bar()
return s:str2

function Foo()
return s:bar() . s:str1

" end of script a

With this script as plugin, I cannot call from command with :echo Foo()

It will report s:str1 and s:str2 not defined.

Any work around?

I need to call the function which returns a string, to include the
string into set 'statusline' in my ~/.vimrc. Here I must call the
function from with vimrc.

Any hint?

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