Haskell requires an explicit type for operations involving input and output.
This way it makes a problem explicit, that exists in every language:
Input and output functions can have so many effects, that it is hard to combine them.
It is hard to test them, because they can in principle depend on every state of the real world.
Thus in order to maintain modularity you should avoid IO whereever possible.
It is too tempting to get rid of IO by
but we want to present some clean techniques to avoid IO.
map putStr vs. putStr concat
STRef instead of IORef, STArray instead of IOArray
Custom type class