I'm learning do expression and Monad using LEARN YOU A HASKELL FOR GREAT GOOD. There's a gcd implementation using tell function makes me confused.
gcd :: Int -> Int -> Writer [String] Int
gcd a b
| b == 0 = tell ["Finished with " ++ show a ] >>= (\_ -> return a)
| otherwise = tell [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)] >>= (\_ -> gcd b (a `mod` b))
gcdResult = gcd 8 3
-- result: WriterT (Identity (1,["8 mod 3 = 2","3 mod 2 = 1","2 mod 1 = 0","Finished with 1"]))
But I am confused by the tell function. When using >>= (\_ -> ...) or >>, the result before will be ignored, so why does the result of tell can be passed to the final result? As my thought, the tell result may be ignored, and the final result will be WriterT (Identity (1,[])).