What is the difference between mapM_ and mapM in Haskell?

Question

I\'ve already checked Hoogle, http://hackage.haskell.org/package/base-4.7.0.1/docs/Prelude.html#v:mapM, which says that mapM_ ignores the results.

However,

Answer 1

mapM_ is useful for executing something only for its side effects. For example, printing a string to standard output doesn't return anything useful - it returns (). If we have a list of three strings, we would end up accumulating a list[(), (), ()]. Building this list has a runtime cost, both in terms of speed and memory usage, so by using mapM_ we can skip this step entirely.

However, sometimes we need to execute side effects and build up a list of the results. If we have a function such as

lookupUserById :: UserId -> IO User

Then we can use this to inflate a list of UserIds to a list of Users:

lookupUsers :: [UserId] -> IO [User]
lookupUsers = mapM lookupUserById