Emacs align-regexp on = but not ==

I am working in Haskell and frequently come across code similar to the following:

func i j | i == j = i
         | otherwise = j

I want to align on the '=' character using align-regexp but don't have the elisp knowhow. I have tried just doing " = " without the quotes, but this inserts an unwanted space character before each '='. I have found a proposed solution here but I can't seem to get that to do anything at all.

Please help me write a function or hard-coded macro that will allow me to set a keybinding for this.