On Jan 7, 2006, at 11:56 AM, Chris Kuklewicz wrote:
This is all about lazy evaluation.
Actually, I understand about lazy evaluation. What I don't
understand is the extent of variable bindings.
If I desugar the code this far:
main = do n <- getArgs >>= return . read . head
main' n
main' n = let p = permutations [1..n]
in do mapM_ (putStrLn . concatMap show) $ take 30 p
putStr $ "Pfannkuchen(" ++ show n ++ ") = "
putStrLn . show $ foldl' (flip (max . steps
0)) 0 p
'p' is a variable bound by a normal 'let.' Why isn't 'p' kept
around until the whole 'in' expression is evaluated? If it were,
then I assume the GC would be obliged to copy everything it pointed to.
In the original version, the author called 'permutations' twice and
didn't create a variable binding.
Cheers, David
--------------------------------
David F. Place
mailto:[EMAIL PROTECTED]
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