thanks for your quick answer. But 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); } will report: parse error on input `let'
Daniel Peebles wrote: > > 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 <z_a...@163.com> 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 >> Haskell-Cafe@haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- View this message in context: http://www.nabble.com/How-to-use-%22bracket%22-properly---tp25953522p25953716.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe