Most docs ([1], [2]) about do-notation syntactic sugar tends to describe following expressions as equivalent: "do { a; b; c }" and "a >> b >> c", but they are not: first one gets de-sugared into "a >> (b >> c)", second one is equivalent to "(a >> b) >> c", because (>>) is declared using infixl. This should not be a problem, monadic law of Associativity states that "(m >>= f) >>= g ≡ m >>= (\x -> f x >>= g)", but this leads to generating different Core output and may lead to different performance (and it does, do { Just 4 ; Just 4 ... } is about 2% faster than Just 4 >> Just 4 >> ... if compiled with -O0, but 13% slower when compiled with -O11) This also leads to lots of fun when your monad breaks Associativity law :) Is there any reasons except for those 13% speed gain for this? [1]: http://en.wikibooks.org/wiki/Haskell/do_Notation [2]: http://book.realworldhaskell.org/read/monads.html#monads.dot