Vim map - to _ and the reverse
 

I am trying to get vim to switch the - and the _ keys. I have got vim to map the - to the _, which is really handy when I am programming. I used

Now, I cannot get it to the reverse, meaning I want to to print a '-' when I would use the key sequence for an '_' (Shift + -). Whatever I map gives me a recursive mapping (which makes sense I guess). Does anyone know how to make this work?

The following will do it:

As you have already done:
imap - _
imap _ -

Type the following two commands as well:

inoremap - _
inoremap _ -