I'm not sure whether I should be impressed or horrified by that postfix hack.
instance Num (MONEY -> Currency) where ...
Any hack with type "Money -> Currency" fits firmly in the "horrific" column.
Even better is to follow up the original usage of that hack, by Lenart Augustsson (pretty much everything he does impresses me), where he embeds a BASIC interpreter in haskell and uses it to make
10 PRINT "hello"
valid :)
I've got a pretty good spreadsheet for calculating Mortgages and payments etc. It's one used by a bank - if anyone wants a copy, mail me.
Upload it to Google docs and share it with everyone.