In Haskell, when we use a do block, how does it figure out which monad to be used? -

we know block syntactic sugar. how figure out monadic context in? assume don't use <- operator anywhere in block.

maybe "practical" examples help:

foo1 =   print 5   return 7 -- print belongs io monad. whole thing in io.  foo2 x =   writetvar x 7   return 11 -- writetvar belongs stm monad. whole thing in stm.  foo3 =   let x = 5   [1, 2, 3, 4] -- last line list expression. whole thing in list monad.  foo4 =   put 7   return 9 -- put in state monad. whole thing in state monad.  foo5 =   x <- magic1   y <- magic2   return (x, y) -- in whatever monad magic1 , magic2 in.  foo6 =   return 13 -- doesn't mention monad. works possible monads!  foo7 abc def =   x <- abc   y <- def   return (x, y) -- runs in whatever monad abc , def run in. -- passing different arguments, can change monad is!