[Haskell-cafe] How to use "bracket" properly ?

[Daniel Peebles]

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_axis at 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 ?
> --
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>
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