Persist state between function calls

[craibuc]

I need to provide a valid sessionid to an API's endpoint.

Towards this end, I created a function:

// script-level variable
Session = null;

// if Session doesn't exist or has expired, generate a new one
GetSession = () as record =>
    if Session=null or DateTimeZone.LocalNow > Session[expires] then 
        () =>
        let
            // get session from API
            Response = Xml.Tables(Web.Contents(...)),
            // transform response
            ...
            // assign data to script-level variable
            Session = [
                expires = DateTimeZone.FromText(data[expires]),
                sessionid = data[sessionid]
            ]
        in
            // return
            Session
    else 
        // use existing 
        Session
;

Unfortunately, the function doesn't work as expected.

What is the recommended way to save a variable's state?