Re: [Haskell-cafe] [Maybe Int] sans Nothings

On Mon, May 23, 2011 at 10:49:55AM -0700, Alexander Solla wrote:

On Mon, May 23, 2011 at 9:20 AM, michael rice wrote:

...

What's the best way to end up with a list composed of only the Just values, no Nothings?

Michael

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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.

Just a minor quibble: note that

filter (not . isNothing)

is slightly preferable since it does not introduce a frivolous equality constraint on the type wrapped by the Maybe. -Brent