Do you want to share what's confusing about mortgage payments?

You have a balance that accrues a monthly interest. Separately, you're told that you're owed a monthly payment. If monthly payment > interest, then the difference is subtracted from the balance. No need for haskell.

Loans can be confusing in double entry accounting for the uninitiated.

On the accounting side you have liabilities:mortgage which would only include the principal. Then for an individual bill you have to debit liabilities:mortgage the principal portion and debit expenses:interest the interest portion.

I’m not sure how mortgage bills usually come but for my auto loan the principal vs interest were not noted on the bill, so i had to calculate via apr myself for each payment for which the interest accrued daily. If i imported those payments from my bank statement or the bill i would not be able to enter the transaction correctly without doing the math.

For calculating interest automatically upon entry based on the rate and payment amount and date I used python iirc as i used beancount, but that was a while ago.