Question

So, it’s been a few months and I’m a bit rusty, but I seem to remember Either b being a Monad

Prelude Control.Monad Data.Either> return "Hello" :: Either String String

<interactive>:0:1:                                                       
    No instance for (Monad (Either String))                              
      arising from a use of `return'                                     
    Possible fix:                                                        
      add an instance declaration for (Monad (Either String))            
    In the expression: return "Hello" :: Either String String            
    In an equation for `it':                                             
        it = return "Hello" :: Either String String                      
Prelude> Right "Hi" == return "Hi"                                    

<interactive>:0:15:                                                   
    No instance for (Monad (Either a0))                               
      arising from a use of `return'                                  
    Possible fix: add an instance declaration for (Monad (Either a0)) 
    In the second argument of `(==)', namely `return "Hi"'            
    In the expression: Right "Hi" == return "Hi"                      
    In an equation for `it': it = Right "Hi" == return "Hi"           

So I’m probably doing something wrong, but I’m not sure what. I’m using GHC 7.2.2.

Answer 1

Import Control.Monad.Trans.Error or Control.Monad.Error.
Either a monad instances are usually used in error handlings.

> return "Hello" :: (Either String String)
Right "Hello"