You left a type in there!
greetABunchOfPeople people
people do
_ sayHello println greetABunchOfPeople people
people do
_ sayHello println greetABunchOfPeople = mapM_ (putStrLn . sayHello)
mapM_ is a function that takes two arguments.
mapM_ lambda list
In the greetABunchOfPeople definition, notice we only gave mapM_ 1 argument instead of 2. That means greetABunchOfPeople would have to take an extra argument for the call to execute.
greetABunchOfPeople $MISSING$ = mapM_ (putStrLn . sayHello) $MISSING$What I love about the Haskell version is it says only exactly what needs to be said.
sayHello - get the hello message for something. putStrLn - send something to stdout mapM_ - Do these things in sequence, I don't care about the result.
Together they say take a list of people, get their greeting strings and print them to stdout in order. It's hard to say it more succinctly without making things confusingly implicit.