They're "in" different Monads. The first one does x <- [...], which means that you're operating in the list Monad instance, and bracket operates in the IO Monad. The second one uses let x = [...] which doesn't have any effect on what Monad you're in, so the whole thing can be in IO.
Note that when you do x <- [1..3]; y <- [4..6] you're going to get all 9 pairs of values from x and y, by the way. Hope this helps, Dan On Mon, Oct 19, 2009 at 1:33 AM, zaxis <[email protected]> wrote: > > winSSQ count noRed noBlue = do { > yesRed <- [1..33] \\ noRed; > yesBlue <- [1..16] \\ noBlue; > bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ > count yesRed yesBlue hd1); > return () > } > will report: > Couldn't match expected type `IO ()' against inferred type `[()]' > In a stmt of a 'do' expression: > bracket > (openFile "ssqNum.txt" WriteMode) > (hClose) > (\ hd1 -> pickSSQ count yesRed yesBlue hd1) > > However, the following works fine: > > winSSQ count noRed noBlue = do > let yesRed = [1..33] \\ noRed > let yesBlue = [1..16] \\ noBlue > bracket (openFile "ssqNum.txt" WriteMode) (hClose) (\hd1 -> pickSSQ > count yesRed yesBlue hd1) > > Why ? > -- > View this message in context: > http://www.nabble.com/How-to-use-%22bracket%22-properly---tp25953522p25953522.html > Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
