Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists

27 Aug 2013

      On Mon, Aug 26, 2013 at 12:05:14PM -0700, Bryan O'Sullivan wrote:
...
On Mon, Aug 26, 2013 at 1:46 AM, Niklas Hambüchen  wrote:
...
This is because sequence is implemented as
sequence (m:ms) = do x <- m
                          xs <- sequence ms
                          return (x:xs)
and uses stack space when used on some [IO a].
This problem is not due to sequence, which doesn't need to add any
strictness here. It occurs because the functions in System.Random are
excessively lazy. In particular, randomIO returns an unevaluated thunk.
I don't understand this.  The same stack overflow occurs with

    tenmil :: Int
    tenmil = 10 * 1000 * 1000

    main :: IO ()
    main = do  
            list <- replicateM tenmil (return ()) :: IO [()] 
            list `seq` return ()

"return ()" is not excessiely lazy, is it?  Could you explain further?

Tom

Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists

Tom Ellis