[Haskell-cafe] IO and lazyness.
haskell at brecknell.org
Tue Mar 6 17:10:48 EST 2007
> The problem is that hGetContents only reads the contents of the file on
> and, without the 'return $!' you don't demand the value until somewhere
> outside of rechf. By this point the hClose has happened and hGetContents
> no access to the file => no lines => no result.
> Using 'return $!' is demanding the value of rech r $ lines f immediately
> hGetContents accesses the file before the hClose.
> hGetContents is implemented using unsafePerformIO so, unless you're very
> careful, you could get other weird behaviours.
For example, if the value for the key being searched is very long (>8k
on my system), it is likely to be truncated. This is because "return $!"
only reduces its argument to WHNF. In other words, it only demands
enough to work out whether the key is present, which means that
hGetContents only gets a chance to return the first block of data
retrieved after the key is found, before the file is closed.
So, instead of "return $! rech r $ lines f", you really need something
case (rech r $ lines f) of
Just s -> return $! Just $! foldr seq s s
Nothing -> return Nothing
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