You don't use another interface when thinking "IO type" instead of "IO monad". It's all about what words you use to explain the types of the interface.
For example, when you write 1 + 2 you are probably thinking of (+) being addition on integers. But a mathematician could also think of (+) as the operator from the group (Z, +, 0). Groups (a concept in abstract algebra) generalize integers, but you do not need to know group theory to do arithmetic. But it's the same operator nevertheless! Just two ways of talking about it.
Similarly, when you write putStrln "Hello " >> putStrLn "world!", the "IO type" way of thinking says: (>>) is an operator that glues two IO actions together. The resulting combined action performs the argument actions sequentially from left to right. The operator has the type IO a -> IO b -> IO b.
But you can think of (>>) in more general terms. It is not meaningful only for the IO type. It is meaningful for any type that happens to be a monad! That insight may be useful eventually, but probably not for a beginner Haskell programmer just wanting to do some IO!
For example, when you write 1 + 2 you are probably thinking of (+) being addition on integers. But a mathematician could also think of (+) as the operator from the group (Z, +, 0). Groups (a concept in abstract algebra) generalize integers, but you do not need to know group theory to do arithmetic. But it's the same operator nevertheless! Just two ways of talking about it.
Similarly, when you write putStrln "Hello " >> putStrLn "world!", the "IO type" way of thinking says: (>>) is an operator that glues two IO actions together. The resulting combined action performs the argument actions sequentially from left to right. The operator has the type IO a -> IO b -> IO b.
But you can think of (>>) in more general terms. It is not meaningful only for the IO type. It is meaningful for any type that happens to be a monad! That insight may be useful eventually, but probably not for a beginner Haskell programmer just wanting to do some IO!