[Haskell-beginners] How to display a time difference?

[Daniel Fischer]

Am Mittwoch, 18. März 2009 11:28 schrieb Colin Paul Adams:
> >>>>> "Yitzchak" == Yitzchak Gale <gale at sefer.org> writes:
>
>     Yitzchak> Colin Paul Adams wrote:
>     >> The code of the following routine is intended to indicate how
>     >> long it takes for the computer to make a move. However the time
>     >> is printed (as very close to zero) long before the move is
>     >> made.
>
>     Yitzchak> You are writing a thunk to the IORef. It only gets
>     Yitzchak> computed later on when you read the value.
>
>     Yitzchak> Try using:
>     >> readIORef game_state_ior >>= evaluate
>     >> . update_interactive_from_move move >>= writeIORef
>     >> game_state_ior
>
>     Yitzchak> evaluate is in Control.Exception.
>
> Still no joy with:
>
> play_move :: IORef Game_state -> IO ()
> play_move game_state_ior = do
>   (_, state, _) <- readIORef game_state_ior
>   putStr "Playing AI: "
>   start_time <- getCurrentTime
>   let move = recommended_move state
>   readIORef game_state_ior >>=
>     evaluate . update_interactive_from_move move >>=
>              writeIORef game_state_ior
>   end_time <- getCurrentTime
>   putStrLn $ show $ (diffUTCTime end_time start_time)

You would have to completely evaluate move. Depending on what type move is, 
that might require much more than what seq or evaluate do.

Perhaps

import Control.Parallel.Strategies

let move = ...
(rnf move) `seq` mofifyIORef (...)

?