What is the best way to extract a Right/Left value in repl without having to write functions that do it every time?
This is ugly:
a = Right 3
getRight x = head $ rights [x]
getRight a -- 3
edit: fromRight requires providing an alternative value, but it may not be trivial to create one with complicated types.
As a one-off, you can write this (pattern matching, or what the imperative programmers call “destructuring assignment”):
ghci> foo = Right 5
ghci> let Right x = foo in x
5
As for getRight, I would define it similarly using pattern matching:
getRight (Right x) = x
Of course, using such things in real-world code is strongly advised against, but no harm no foul in the repl. 
You can even do:
ghci> foo = Right 5
ghci> let Right x = foo
ghci> x
5
That way you can use x multiple times.
You don’t even need to use let:
ghci> foo = Right 5
ghci> Right x = foo
ghci> x
5
Old habit, I guess!
I was curious about when the requirement to use let was removed, and I managed to find the ticket (I think). Apparently it was removed in 8.0.1!