[Haskell-cafe] Deadlock in real number multiplication (Was:
Where's the problem ?)
phil at kantaka.co.uk
Thu Jul 5 05:32:19 EDT 2007
On Thu, Jul 05, 2007 at 10:49:17AM +0400, Miguel wrote:
>R> Another interesting thing I've discovered is:
>Prelude>> length [1..1000000]
>Prelude>> Data.List.genericLength [1..1000000]
>R> *** Exception: stack overflow
>R> Maybe there is something wrong with Integer ?
>No, there is something wrong with genericLength:
>Prelude> Data.List.foldl (+) 0 $ map (const 1) [1..1000000] :: Int
>*** Exception: stack overflow
>Prelude> Data.List.foldl' (+) 0 $ map (const 1) [1..1000000] :: Integer
genericLength :: (Num i) => [b] -> i
genericLength  = 0
genericLength (_:l) = 1 + genericLength l
So genericLength is lazily building up unevaluated (+) expressions and
running out of stack space.
Is there a good reason for genericLength to be lazy?
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