`let' for comprehension

[Serge D. Mechveliani]

Who can advise, please, 

is this for sure that the program

   \ x ->  [f y | y <- g x]

is not more expensive in computation than

   \ x ->  let  ys = g x  in  [f y | y <- ys]
?
May the difference depend on Haskell implementation?

(I suspect that compilers convert one of these programs to another).

Thank you in advance for explanation.

-----------------
Serge Mechveliani
mechvel at botik.ru