[Haskell-cafe] [Maybe Int] sans Nothings

[Alexander Solla]

On Mon, May 23, 2011 at 9:20 AM, michael rice <nowgate at yahoo.com> wrote:

> What's the best way to end up with a list composed of only the Just values,
> no Nothings?
>
> Michael
>
> ==========================
>
> import Control.Monad.State
> import Data.Maybe
>
>
> type GeneratorState = State Int
>
> tick :: GeneratorState (Maybe Int)
> tick = do n <- get
>           if ((n `mod` 7) == 0)
>             then
>               return Nothing
>             else do
>               put (n+1)
>               return (Just n)
>
> {-
> *Main> evalState (sequence $ replicate 9 tick) 1
> [Just 1,Just 2,Just 3,Just 4,Just 5,Just 6,Nothing,Nothing,Nothing]
> -}
>
>
There's a library function for it, but also:

> filter ((/=) Nothing)

is readable enough.
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