[Haskell-beginners] Printing the result of a function evaluation

[Stephen Blackheath [to Haskell-Beginners]]

Travis,

because 'speedTest (read $ head args) `seq` return ()' is semantically
equivalent to 'return ()' whose type is IO () therefore 'v <- a' assigns
() to v.

'time (return $! speedTest (read $ head args))' is better...

however, it's still not going to do what you want (time how long it
takes speedTest to evaluate, I'm assuming), because (speedTest (read $
head args)) will only get evaluated once.

The only way  to actually time how long speedTest takes is to pass a
function e.g. (Int -> IO t) to 'time', and have 'time' pass it a dummy
argument, so that GHC can't optimize the evaluation of speedTest out.


Steve

Travis Erdman wrote:
> Why oh why doesn't this work as I expect, and what is the simplest fix? 
> 
> speedTest 4 = 44
> 
> time :: (Show t) => IO t -> IO t
> time a = do
>     start <- getCPUTime
>     v <- a
>     end   <- getCPUTime
>     let diff = (fromIntegral (end - start)) / (10^12)
>     print v -- WHY DOESNT THIS WORK?  ALWAYS PRINTS () ....
>     printf "Computation time: %0.3f sec\n" (diff :: Double)
>     return v
> 
> main :: IO ()
> main = do
>     args <- getArgs
>     time $ speedTest (read $ head args) `seq` return ()
> 
> 
> 
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