Is the monadic IO construct in Haskell just a convention?

Question

Is the monadic IO construct in Haskell just a convention or is there is a implementation reason for it?

Could you not just FFI into libc.so instead to do your IO, and

Answer 1

Could you just FFI into libc.so instead to do IO and skip the IO Monad thing?

Taking from https://en.wikibooks.org/wiki/Haskell/FFI#Impure_C_Functions, if you declare an FFI function as pure (so, with no reference to IO), then

GHC sees no point in calculating twice the result of a pure function

which means the the result of the function call is effectively cached. For example, a program where a foreign impure pseudo-random number generator is declared to return a CUInt

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign
import Foreign.C.Types

foreign import ccall unsafe "stdlib.h rand"
  c_rand :: CUInt

main = putStrLn (show c_rand) >> putStrLn (show c_rand)

returns the same thing every call, at least on my compiler/system:

16807
16807

If we change the declaration to return a IO CUInt

{-# LANGUAGE ForeignFunctionInterface #-}

import Foreign
import Foreign.C.Types

foreign import ccall unsafe "stdlib.h rand"
  c_rand :: IO CUInt

main = c_rand >>= putStrLn . show >> c_rand >>= putStrLn . show

then this results in (probably) a different number returned each call, since the compiler knows it's impure:

16807
282475249

So you're back to having to use IO for the calls to the standard libraries anyway.