[Haskell-beginners] foldl on Bool:s

Reading http://learnyouahaskell.com/higher-order-functions I understand that with the function sum' :: (Num a) => [a] -> a sum' = foldl (+) 0 the call ghci>>> sum' [1,2,3] will be evaluated as 0 + 1 + 2 + 3 = 6 But what about the function elem' :: (Eq a) => a -> [a] -> Bool elem' y ys = foldl (\acc x -> if x == y then True else acc) False ys and calling it with ghci>>> elem' 3 [1,2,3] How is that evaluated to True by foldl in elem'? Thanks in advance for any explanation to this!