[Haskell-beginners] how to skip pattern match error when applying a mapM_
fa-ml at ariis.it
Tue Jan 17 14:19:46 UTC 2017
On Tue, Jan 17, 2017 at 02:03:02PM +0000, PICCA Frederic-Emmanuel wrote:
> Here a reduction of my problem
> values :: IO [IO (Maybe Int)]
> values = do
> let v = [Just 1, Just 2, Just 3, Nothing, Just 5, Nothing, Just 7] :: [Maybe Int]
> return $ map return v
> main :: IO ()
> main = do
> vs <- values
> nvs <- mapM_ go vs
> print nvs
> go :: IO (Maybe Int) -> IO Int
> go v' = do
> Just v <- v'
> return v
`Just v <- v'` doesn't silently skip Nothing values, but it's
a full fledged pattern match (and one reason why I dislike `do
notation` as a syntactic sugar).
A way to solve the problem is to take advantage of `sequence`
and `catMaybes` (from `Data.Maybe`).
λ> :t sequence
sequence :: (Monad m) => [m a] -> m [a]
-- I cheated a bit on the signature, but the gist
-- of it is: from a list of monadic actions, to
-- one monadic action returning a list of results.
λ> :t catMaybes
catMaybes :: [Maybe a] -> [a]
With that your main gets simpler:
main :: IO ()
main = do vs <- values -- vs :: [IO (Maybe Int)]
sv <- sequence vs -- sequence vs :: IO [Maybe Int]
-- sv :: [Maybe Int]
print (M.catMaybes sv)
Does this help?
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