*Main> eval exp1
Just 7
*Main> eval exp2
Nothing
*Main> eval exp1
Just 7
*Main> eval' exp1
Just 7
*Main> eval' exp2
Nothing
*Main> :t (>>=)
(>>=) :: Monad m => m a -> (a -> m b) -> m b
*Main> :r
[1 of 1] Compiling Main             ( lec10.hs, interpreted )
Ok, one module loaded.
*Main> eval'' exp2
Nothing
*Main> eval'' exp1
Just 7
*Main> zip''' [1, 2,3] "abcd"
[(1,'a'),(1,'b'),(1,'c'),(1,'d'),(2,'a'),(2,'b'),(2,'c'),(2,'d'),(3,'a'),(3,'b'),(3,'c'),(3,'d')]
*Main> :r
[1 of 1] Compiling Main             ( lec10.hs, interpreted )
Ok, one module loaded.

*Main> pairing'' [1, 2,3] "abcd"
[(1,'a'),(1,'b'),(1,'c'),(1,'d'),(2,'a'),(2,'b'),(2,'c'),(2,'d'),(3,'a'),(3,'b'),(3,'c'),(3,'d')]
*Main> pairing''' [] "abcd"
[]
*Main> pairing''' "abcd" []
[]
*Main> :t (1, [])
(1, []) :: Num a1 => (a1, [a2])
*Main>