[Haskell-cafe] runStateT execution times measurement baffling

[Albert Y. C. Lai]

On 11-10-20 01:38 PM, thomas burt wrote:
> I've been trying to measure execution time for some code I'm running
> with the StateT monad transformer.
>
> I have a function f :: StateT MyState IO a
>
> Now, I measure the time it takes to run an invocation of this function
> from beginning to end, i.e.
>
> f = do
>    t0 <- getCurrentTime
>    stuffToDo
>    t1 <- getCurrentTime
>    liftIO $ putStrLn (show $ diffUTCTime t1 t0)
>
> And also measure like this:
>
> g :: IO
> g = do
>    t0 <- getCurrentTime
>    (val,newState) <- runStateT f initialState
>    t1 <- getCurrentTime
>    putStrLn  $ "outside: " ++ (show $ diffUTCTime t1 t0)

Where can I find stuffToDo?

How do you get away with getCurrentTime (as opposed to liftIO 
getCurrentTime) inside f? Does not type-check.

How much should I trust this code?