Ask HN: How to approach this problem functionally

Here is a logic that I have to implement in an in-house application that manages attendance, leaves, etc. The following is one of the workflows in leave application processing.

    if (attendance has been marked for any session of the leave duration) {
        perform exception processing // excluded for simplicity
        return
    }

    if (applied leave duration overlaps (partly or fully) with any other currently-valid leave application) {
        reject and return
    }

    switch (leave type) {
    case annual:
        if (not enough annual leave balance) {
            reject and return
        }
        // more annual condition checks

    case carry-forward:
        if (not enough carry forward leave balance) {
            reject and return
        }
        // more carry-forward condition checks

    case paternity, maternity:
        branch out to eligibility and extent of availability processing // excluded for simplicity

    case compensatory-off:
        if (date against which compensation is availed was working) {
            reject and return
        }
        if (employee did not work on that day) {
            reject and return
        }
        if (that date is already used for compensatory off) {
            reject and return
        }
        // more compensatory checks

    case loss-of-pay:
        // several loss-of-pay checks

    // several more varieties
    }

    load the holiday calendar based on employee's location
    load the shift details for the employee
    calculate effective duration of leave

    insert / update all applicable records
    write audit entries and logs

    send response back

Significantly simplified logic above excludes things like leave duration crossing year boundary (some counters get reset; new leave quotas may become available).

I look forward to guidance on how to approach this from a functional programming perspective. Thanks.