Implementing "Pierre the Tight-rope" Walker from LYAH
I don't see glaring flaws. A couple notes though:

-You could combine the addLeft/Right functions and pass in a tuple representing how many birds you want to add ( (0,2) for 2 right ); the functions are almost identical.

-I've found where bindings to read neater if I have more then 2 (personal choice). 

-AFAIK, the Error Monad /= the Either Monad.