Implementation of null function

Question

I have to learn Haskell for university and therefor I\'m using learnyouahaskell.com for the beginning.
I always used imperative languages so I decided to practice Haskell

Answer 1

You can't just turn the clauses around with your solution:

mNull' :: [a] -> Bool
mNull' _  = False
mNull' [] = True

this will always yield False, even if you pass an empty list. Because the runtime doesn't ever consider the [] clause, it immediately sees _ matches anything. (GHC will warn you about such an overlapping pattern.)

On the other hand,

null' :: [a] -> Bool
null' (_:_) = False
null' []    = True

still works correctly, because (_:_) fails to match the particular case of an empty list.

That in itself doesn't really give the two explicit clauses an advantage. However, in more complicated code, writing out all the mutually excluse options has one benefit: if you've forgotten one option, the compiler can also warn you about that! Whereas a _ can and will just handle any case not dealt with by the previous clauses, even if that's not actually correct.